Detection of printing and coating media

ABSTRACT

Method and apparatus are described for determining if an optically variable material (OVM) is present in or on a surface, typically of a security document such as a banknote. In some embodiments the particular type of OVM is identified. In other embodiments a pass or fail signal is generated depending on whether a specific OVM is found on the surface. The surface is subjected to at least two different wavelengths of illuminating light at a particular angle to the surface and the intensity of the light reflected and/or scattered light from the surface is determined using photodiodes. The analogue output from the photodiodes may be digitised and the digital values processed according to one or another of different algorithms described, so as to produce an output signal whose value can be compared with one or more stored reference values in a look-up table, to produce a decision signal. Comparison of the analogue photodiode output signal values against a look-up table can be used to indicate the concentration of the OVM on the surface. Methods and apparatus are described for document authentication and identification, coating quality control and document sorting, by employing the methods and apparatus described.

FIELD OF THE INVENTION

This invention concerns methods and apparatus for detecting the presencein or on a surface, particularly for identifying the presence ofdifferent printing or coating media and particularly detecting andidentifying optically variable materials in or on printed or coatedsurfaces. The invention is especially concerned with the detection ofoptically variable materials on security documents such as, but notlimited to banknotes, passports, driving licences, identity cards, bankand credit cards, security passes, and the like. Typically materials areinks, dyes and varnishes.

BACKGROUND OF THE INVENTION

In the security document printing industry, it is commonplace to provideareas on a document with specific optical characteristics which can beidentified by illumination in a predetermined way at a particularwavelength and/or polarisation. Examples include the use of hologramsand gratings incorporated onto such documents. It has been proposed toincorporate in or on such a document one or more regions of an OpticallyVariable Material, henceforth referred to as OVM.

An optically variable material (OVM) can comprise any material whichchanges colour depending on the angle at which a surface containing thematerial is viewed, and includes pearlescent materials, iridescentmaterials, liquid crystalline materials and OV inks such as sold underthe trade name OVI by Sicpa SA of Lausanne, Switzerland.

It is a property of an OVM that a region printed or coated using such amaterial will appear differently coloured depending on the viewing angleand the angle of illumination. In one example if a flat printed surfacecontaining one particular OVM is illuminated by white light at an angleof 45° to the normal, the OVM appears purple to the human eye when backscattered light is viewed, orange when direct (specular) reflection isviewed, and green if viewed along the surface of the paper, that is at aglancing angle.

OVM inks generally fall into two categories, non-pearlescent andpearlescent. The optical characteristics of such inks are different andboth again differ from the optical characteristics of a non-OVM(metallic) ink.

It is an object of the present invention to provide a method andapparatus for detecting the presence of an OVM in or on a surface,typically the surface of a security document.

It is another object of the present invention to provide a method andapparatus for detecting, and identifying the type of, OVM in or on asurface, typically the surface of a security document.

It is another object of the present invention to provide a method andapparatus for illuminating, and responding to reflected light from, asurface, and which is adapted to generate a signal indicative ofmaterial present in or on the illuminated surface.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention a method ofdetecting the presence of a non-pearlescent OVM in or on a surface,comprises the steps of illuminating the surface at a first angle to thesurface and detecting and determining the frequency spectrum ofscattered light in two different directions from the surface, onedirection subtending an angle to the surface which is substantiallydifferent from the said first angle and is substantially parallel to theplane of the surface, and the other direction subtending an angle to thesurface which is substantially closer to the said first angle than thesaid one direction.

In a preferred method the angle of the one direction to the surface (thesecond angle) is in the range 1° to 15° and the angle made by the otherdirection to the surface (the third angle) is within 10° of the saidfirst angle.

In a particularly preferred method the said second angle is 10° and thesaid third angle equals the said first angle.

According to a second aspect of the present invention a method ofdetecting the presence of a pearlescent OVM in or on a surface comprisesthe steps of illuminating the surface at a first angle and firstlydetecting and determining the frequency spectrum of substantially directspecular reflection from the surface, and secondly detecting anddetermining the frequency spectrum of scattered light leaving thesurface at an angle which is different from that at which directspecular reflection occurs.

In one method incorporating this second aspect of the invention thesecond detection is of forwardly scattered light.

Typically the forwardly scattered light is detected at an angle to thesaid surface which is in the range 1° to 15°, preferably 10°.

In another method incorporating this second aspect of the invention thesecond detection is of back scattered light.

Typically the back scattered light is detected at an angle within 10° ofthe direction in which illuminating light is projected towards thesurface, and preferably is detected at substantially the same angle asthat which the illuminating light makes to the said surface.

In methods incorporating either the first or second aspect of theinvention the two detections may be performed in succession one afterthe other or simultaneously.

Where the detections are performed in succession a single detector maybe employed, which is moved between the two positions to allow lightwhich is being reflected from or scattered by the surface in thedifferent directions of interest to be intercepted.

Alternatively where a plurality of detectors is provided each fixed inposition to intercept reflected or scattered light as appropriate, thedetectors may be separately interrogated either one after the other, toprovide a succession of intensity values or simultaneously to provide acorresponding plurality of intensity values.

The spectral determination of the light incident on the or each detectormay be performed by the detector if a suitable photo-sensitive elementor a combination of one or more filters and at least one photo-detectoris employed for the or each detector which will supply different signalsor different values of a parameter of a signal, depending on thewavelength of light incident thereon.

The illuminating light may be white light or more preferably the lightis made up of two or more distinct monochromatic components havingdifferent (known) wavelengths.

According to a third aspect of the present invention a method ofdetermining if a surface contains a specific OVM comprises the steps ofilluminating the surface using light containing two substantiallymonochromatic components of wavelength λ₁ and λ₂, detecting theintensity of scattered light from the surface at the two scatteringangles Ø₁ and Ø₂ selected according to the OVM of interest, computingthe magnitude of the difference between the intensity values for λ₁light at Ø₁ and Ø₂, less the difference between the intensity values forλ₂ light at Ø₁ and Ø₂, and generating an output signal indicating thepresence or absence of the specific OVM depending on the computeddifference magnitude relative to a predetermined value.

Typically the computed difference value magnitude is compared with thepredetermined value to generate a first output signal value indicatingthe presence of the specific OVM, if the computed magnitude is at leastas great as the predetermined value, or a second output signal valueindicating that the specific OVM has not been detected, if the computedmagnitude is less than the predetermined value.

In a method incorporating the said third aspect of the invention, thesurface may be illuminated by the two monochromatic componentssimultaneously, or preferably separately first with monochromatic lightof one wavelength λ₁ and then with monochromatic light of the secondwavelength λ₂.

Detection may be performed by a single detector which is moved betweentwo positions so as to receive light reflected/scattered from thesurface first at one and then the other of the two angles Ø₁ and Ø₂, ormore preferably by means of two detectors which are positioned so as toreceive light from the surface along the directions dictated by Ø₁ andØ₂.

Where one detector is employed the values of signals from it at the twodifferent positions may need to be modified to take account of anyinherent differences in intensity of the originating illuminationsincident on the surface due for example to different intensity levelsand/or any misalignment of the sources of the λ₁ and λ₂ light.

Where two detectors are employed the values of the signals from one orboth detectors may also need to be modified to take account of anyinherent differences in the responses of the two detectors to light ofgiven intensity incident thereon, and any misalignment of the detectors.

A method incorporating the said third aspect of the invention thereforepreferably includes a calibration procedure in which the light isprojected towards and the reflected/scattered light is received from, anon OVM containing matt white surface.

According to a fourth aspect of the invention, in a method incorporatingthe said third aspect of the invention the absolute value of thecomputed difference value may be compared with a range of possiblevalues, the different values in the range corresponding to differingconcentrations of the specific OVM in or on the surface under test.

Preferably the light projected onto the surface is collimated.

Where two detectors are employed as is preferred, the signals from eachdetector may be gated or addressed in synchronism with the changingwavelength of the illuminating light, so that during each gating oraddressing period the wavelength of the incident light is known andthere is no light of the other wavelength present to confuse matters.

The values of λ₁ and λ₂ and Ø₁ and Ø₂ are selected by reference toinformation obtained by interrogating light reflected/scattered by asurface containing the OVM of interest at different angles to thesurface, as described herein. By recording the results of such tests atthe different angles, two wavelengths and two angles which give bestreflection/scatter for those wavelengths, can be determined as beingunique to surfaces containing that OVM.

Calculation of the difference magnitude in a way which compensates forvariations in intensity between one wavelength component and the other,variations due to misalignment, and variations between detectorresponses, is achieved by calculating the difference magnitude M usingequation (1).M=|(K ₁ R ₁ −K ₂ R ₂ −A*(K ₁ G ₁ −K ₂ G ₂)|  (1)where R and G represent the reflectance signal intensity valuesoutputted by the photo-detectors at the illuminations of λ₁ and λ₂respectively, the subscripts of R and G denoting measurements made bythe two detectors at the scattering angles of Ø₁ and Ø₂.

Since adjustment of the relative intensities, alignment of the twosources of λ₁ and λ₂ illumination, and alignment of the detectors willcause variation in the magnitude of the detector output signals, thecalibration constants K₁ and K₂ are included to allow for misalignmentand differences in the photo-detector responses at Ø₁ and Ø₂ scatteringangles.

The calibration constants K₁ and K₂ may be set by adjusting the gains ofthe detector output signal amplification.

The scalar constant A normalises for differences in the detectorresponses and alignment relative to the λ₁ and λ₂ illuminations.

A method of calibrating the detector involves inserting a plain mattwhite surface in place of the surface to be tested, and adjusting thevalues of K₁ and K₂ so that K₁R₁=K₂R₂=1. The scalar A is then adjustedto give M=0 with the matt white surface.

The generation of a YES/NO signal is typically achieved by comparing thecomputed value of M with a predetermined value T, itself derived bycomputing M from a surface containing a known minimum concentration ofthe OVM of interest.

Preferably, a plurality of different values of M are computed using aplurality of samples each containing a different (known) concentrationof the particular OVM and storing same to form a range of values of Tfor comparison with computed values of M from surfaces having an unknownconcentration of the OVM thereon.

Substitution of the white surface with a sample having a printed orcoated surface allows the sample surface to be checked for the presenceof the particular OVM. If the OVM is present, the magnitude of M willequal or exceed the predetermined threshold value T, and if not, themagnitude of M will be less than the threshold.

Since the magnitude of M will vary with quantity of OVM present (i.e.concentration for a given constant area), the range of possible valuesfor M, for a given range of samples each containing the same OVM, can bethought of as comprising a “grey scale” output representative of thequantity of OVM ink present, and to this end a look up memory may beprovided for each OVM containing different values of T for differingconcentrations of that OVM.

The invention also lies in apparatus adapted to perform any of themethods so far described herein as incorporating or comprising theinvention or an aspect of the invention.

According to the invention there is provided apparatus by which anoutput signal is generated indicative of the presence of a specific OVMin or on a surface under test comprising:

-   1. A light source which produces and projects along a projection    axis monochromatic light at each of two wavelengths λ₁ and λ₂    selected according to the specific OVM of interest;-   2. Means for locating the surface under test to receive the light    with the projection axis at a specific angle to the surface;-   3. Two photodetectors, the first of which is located to receive    light reflected at a first scattering angle Ø₁, and the second of    which is located so as to receive light reflected at a second    scattering angle Ø₂, from the surface, each photodetector producing    an analogue signal indicative of the intensity of light incident    thereon;-   4. Means for adjusting the intensities of the λ₁ and λ₂    illuminations;-   5. Means for amplifying the signals from the photodetectors;-   6. Means for computing the value of the magnitude of the difference    between the amplified intensity values for λ₁ light    reflected/scattered from the surface at Ø₁ and Ø₂ less the    difference between the amplified intensity values for λ₂ light    reflected/scattered from the surface at Ø₁ and Ø₂;-   7. Means for generating an output signal dependent on the magnitude    of the computed difference value to indicate the presence of the    material on the surface.

A YES/NO output signal may be obtained by comparing the output signalwith a reference.

Preferably the illumination intensities and the gains of the amplifyingmeans are adjusted to calibrate the apparatus, during a calibrationstep;

It is preferable for the light source to comprise a pair of LED's, onewhich emits near monochromatic light at or near λ₁ and the other at ornear λ₂, and the light from the two LED's is projected along a commonaxis.

Preferably the angle of incidence of the light upon the surface shouldbe at or close to 45 degrees.

Preferably projected light is collimated.

Preferably the photo-detector means comprises a pair of photo-diodes.

It is preferable for each photo-detector to be associated with a lensfor focusing light onto the diode, and that this lens should have anaperture optimised to limit the angular range of scattered lightincident on the detector, but allowing appropriate and practicablelevels of light through.

It is preferable for the apparatus to include a pulsed power supply forthe two LED's such that the two LED's are operated alternately. Therepetition rate of the two LED's may be in the range 1 KHz up to 1 MHzor higher, limited only by the time response of the photo-detectors.

The apparatus may be electronically hard wired or may supply signals to,and be controlled by, a computer with a suitable interface and dataacquisition card. In the case of a general purpose computer, the chosenmethod is performed by suitably programming the computer to makemeasurements on and/or compute differences and/or ratios between, theoutput signal values to produce a classifying signal for a surface undertest.

Typically the interface or the apparatus includes analogue signalamplifying means and an analogue to digital converter for supplyingdigital signals to the data acquisition card.

Although the method and apparatus so far described enables the presenceor absence of specific OVM to be distinguished, according toillumination wavelengths and scattering angles used, and for OVM to bedistinguished from certain other OVM and non-OVM, and can produce a“grey-scale” output representative of the amount or quality of an OVMpresent on a surface, the invention also provides an alternative methodand apparatus to distinguish between pearlescent OVM, non-pearlescentOVM, and non-OVM in or on a surface.

According therefore to a fifth aspect of the invention, there isprovided a method of determining if a surface contains a specificmaterial comprising the steps of:

-   (1) detecting the intensity of the light reflected or scattered by    the surface at a third angle Ø₃, such that Ø₁ corresponds to back    scattered light, Ø₂ corresponds to a near specular reflection, and    Ø₃ corresponds to light leaving the surface at a glancing scattering    angle,-   (2) generating three output signals by computing hue ratios h_(Ø1),    h_(Ø2) and h_(Ø3), using the pairs of intensity values from each of    the three detectors for the two monochromatic λ₁ and λ₂ components    of illumination, and-   (3) comparing the computed hue ratios with a predetermined group of    three stored values, obtained by experiment, to generate a final    output signal whose value depends on the comparison.

Preferably the intensity values from the detectors are adjusted tocompensate for background light by measuring and storing the detectoroutput signal value when a surface is present but no λ₁ or λ₂illumination is incident thereon.

In the fifth aspect of the invention the angles are preferably selectedas being Ø₁=45°, Ø₂=90° and Ø₃=110°.

It has been found that by carefully selecting not only the values forØ₁, Ø₂ and Ø₃, but also carefully selecting the values of λ₁ and λ₂, itis possible to employ the method to indicate not only if a particularOVM is present, but in the case of surface coatings and inks, toindicate whether the coating or ink is a pearlescent OVM, anon-pearlescent OVM, or a non-OVM substance, and in the case of thelatter to distinguish between matt and glossy coatings.

Preferred values for λ₁ and λ₂ which enable such identification to occurare:λ₁=654 nm and λ₂=574 nm.

Groups of values for the three hue ratios can be obtained by performingthe method according to the fifth aspect of the invention and noting andstoring in groups of three, the three hue ratio values for surfacescontaining different (known) coatings or inks in a look-up memory, eachgroup of three values having stored therewith or linked thereto dataindicating the material producing those three values. This look-upmemory can then be employed to identify an unknown material in or on asurface subsequently subjected to the method.

The invention can be employed to distinguish between and therebyidentify matt inks, glossy inks, OVM inks, and pearlescent OVM inks andaccording to the invention a method of determining the material presentin or on a surface for which three hue ratios h_(Ø1), h_(Ø2) and h_(Ø3)are determined according to the fifth aspect of the invention as betweena matt ink, a glossy ink, an OVM ink, and a pearlescent OVM ink bychecking the hue ratios using the following criteria:

-   (i) if h_(Ø1), h_(Ø2) and h_(Ø3) are substantially constant with    scattering angle (and have a value which is not tending to unity)    this indicates a matt ink,-   (ii) if h_(Ø2) tends to unity and h_(Ø1) and h_(Ø3) are    significantly different from unity, this indicates a glossy ink,-   (iii) if h_(Ø1), h_(Ø2) and h_(Ø3) decrease with increasing    scattering angle, and the decreases tend to be substantial, this    indicates an OVM ink,-   (iv) if specular reflection produces a more saturated colour    resulting in the h_(φ2) ratio diverging from unity, this indicates a    pearlescent OVM ink.

Preferably the angle of incidence of the illuminating light on thesurface is at or near 45°.

Preferably the angles of detection are selected as: Ø₁=45°, Ø₂=90° andØ₃=110°

Preferably also the illuminating light is collimated.

The invention also lies in apparatus adapted to perform the modifiedmethod provided by the fifth aspect of the invention, comprising:

-   (1) a light source which produces and projects along a projection    axis monochromatic light at each of two wavelengths λ₁ and λ₂,-   (2) platform means for locating a surface under test to receive the    light with the projection axis at a specific angle to the platform,-   (3) three photo-detectors located relative to the platform so as to    separately receive reflected/scattered light from a surface thereon    at three different angles Ø₁, Ø₂, and Ø₃, where Ø₁ corresponds to    back scattered light, Ø₂ to near specular reflection and Ø₃ to light    leaving the surface at a shallow angle (a glancing scattering    angle),-   (4) means for adjusting the intensities of the λ₁ and λ₂ components    of illumination,-   (5) means for amplifying the signals from the photo-detectors,-   (6) means for computing the ratio of the response of each    photo-detector to the two different wavelengths in the    reflected/scattered light incident thereon after taking background    light into account,-   (7) comparator means for comparing the three ratio values so    obtained with at least one set of three stored values, and    generating an output signal dependent on the comparison.

The comparator means may generate either a YES/NO signal in response tothe comparison depending on whether or not the three computed values aresimilar to the three stored values, or an identification signalindicating which of a plurality of groups of stored values (eachcomprising a group of three such values) the three computed values mostclosely correspond.

Preferably the photo-detector output signals are adjusted for backgroundillumination before the hue ratios are computed. Background intensitylevel is obtained by noting each photo-detector output signal value withthe surface in place but when no λ₁ or λ₂ illumination is present. Thisvalue may be deducted from subsequent output signals from thatphoto-detector obtained when the surface is illuminated by λ₁ and λ₂illumination.

The apparatus may be electronically hard wired or may supply signals to,and be controlled by, a computer with a suitable interface and dataacquisition card. In the case of a general purpose computer, the chosenmethod is performed by suitably programming the computer to makemeasurements on and/or compute differences and/or ratios between, theoutput signal values to produce a classifying signal for a surface undertest.

Typically the interface or the apparatus includes analogue signalamplifying means and an analogue to digital converter for supplyingdigital signals to the data acquisition card.

According to a sixth aspect of the invention a further method ofidentifying the presence of a particular type of material on a printedor coated surface, comprises the steps of:

-   1. Illuminating the surface at a pre-set angle to the surface with    substantially monochromatic light at three wavelengths λ₁, λ₂ and λ₃    selected in accordance with the particular type of material,-   2. Detecting light reflected from the surface at three different    angles Ø₁, Ø₂ and Ø₃, one of which Ø₂ corresponds to a near specular    reflection and the other two of which are selected in accordance    with the particular type of material and are at or near to those at    which the illumination wavelengths give good reflectance changes for    the particular type of material.-   3. Computing 3 hue values from the intensity values determined by    each detector for each of the three monochromatic illumination    components λ₁, λ₂ and λ₃, thereby to produce 9 hue values relating    to the surface,-   4. Comparing the 9 values so obtained with 9 stored hue values,    obtained by performing the method on a surface containing some of    the particular type of material,-   5. Generating a final output signal whose value depends on the    comparison.

The final output signal may be in binary format and posses one valueonly if identity or near identity is obtained by the comparison, therebyindicating that the particular type of material is present.

Preferably the hue values are computed after taking into account andadjusting the photo-detector output signals for any backgroundillumination. This may be achieved by noting the photo-detector outputsignals with the surface present but in the absence of any λ₁, λ₂ or λ₃illumination.

Preferably the comparison is performed by calculating the nearestneighbour classifier using the 9 stored hue values for the particularmaterial.

The nearest neighbour classifier may be computed by summing the squaresof the differences between the computed hue values and stored hue valuesand comparing the sum with a threshold. In the case of identity thevalue of the sum is zero and near identity situations can be identifiedif the sum value is less than a small numerical value selected for thethreshold.

Computation of the nine hue values r_(φ1) g_(φ1) etc., for the threedetectors receiving reflected/scattered light at the angles Ø₁, Ø₂ andØ₃ at the wavelengths λ₁, λ₂ and λ₃, each of which produces an intensityvalue in the detector output of R_(φ1), G_(φ), or B_(φ)respectively, andwhere the background illumination produces an intensity value D_(φ1),D_(φ2) etc., in the detector output, is achieved by using the equations:r _(φ1)=(R _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))g _(φ1)=(G _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))b _(φ1)=(B _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))r _(φ2)=(R _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))g _(φ2)=(G _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))b _(φ2)=(B _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))r _(φ3)=(R _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3))g _(φ3)=(G _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3))b _(φ3)=(B _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3))

-   -   where R_(φ1,2,3) are λ₁ illuminated signal from detectors at        scattering angles of Ø₁, Ø₂, and Ø₃ respectively; G_(φ1,2,3) and        B_(φ1,2,3) are the same but for λ₂ and λ₃ illumination        respectively.

Calibration may be achieved by performing the method using a surfacecontaining the particular material of interest and computing the 9values r_(φ1), g_(φ1) etc., and storing the computed values as α₁, α₂,α₃, β₁, β₂, β₃, γ₁, γ₂, γ₃.

Nearest neighbour computation may be performed using the followingformula:

D = (r_(ϕ 1) − α₁)² + (g_(ϕ 1) − β₁)² + (b_(ϕ 1) − γ₁)²+    (r_(ϕ 2) − α₂)² + (g_(ϕ 2) − β₂)² + (b_(ϕ 2) − γ₂)²+    (r_(ϕ 3) − α₃)² + (g_(ϕ 3) − β₃)² + (b_(ϕ 3) − γ₃)²

The method just described may be used to distinguish between OVM's,non-OVM's and matt and glossy inks, and also enables small variations inOVM quality or quantity to be detected. Tolerance to such variations canbe increased by altering the value of the threshold, albeit at theexpense of the likelihood of miss-classification.

Multiple material type classifications can be performed by pre-storingdifferent α, β, γ values for different pre-measured samples andperforming nearest neighbour threshold classification using each of thestored α, β, γ value sets, until the lowest value of the sum isobtained, indicating the best match.

Apparatus adapted to perform the method of the sixth aspect of theinvention comprises:

-   (1) three monochromatic light sources producing light of λ₁ λ₂ and    λ₃ wavelengths, the particular wavelengths being selected in    relation to the material to be identified,-   (2) means for projecting the light at a particular angle towards a    support means on which a sheet of material the surface of which is    to be investigated can be laid, the angle being selected in relation    to the materials to be identified,-   (3) three photo-detectors arranged relative to the support means to    receive reflected/scattered light along three different directions    therefrom, the directions being selected in relation to the material    to be identified;-   (4) computing means adapted to receive intensity signals from the    three detectors and compute therefrom nine hue values corresponding    to ratios of intensity signal values and combinations of such signal    values, from each detector;-   (5) memory means adapted to store at least one set of nine hue    values obtained by using a sheet of material containing at least in    or on the surface thereof the material which is to be looked for in    other surfaces,-   (6) comparison means for comparing computed and stored hue values to    generate a binary output signal one value of which is generated only    if identity or near identity exists between the computed and stored    hue values.

Preferably the hue values are computed after taking into account, andadjusting the photo-detector output signals for, any backgroundillumination. This may be achieved by noting the photo-detector outputsignals with the surface present but in the absence of any λ₁, λ₂ and λ₃illumination.

The comparison means may comprise a computing means adapted to computethe sum of the squares of the differences between the computed andstored hue values.

Preferably the projection angle is 45°.

Preferably collimating means is provided to collimate the projectedlight.

Preferably each photo-detector comprises a photo-diode.

Preferably lens means is provided for focusing reflected/scattered lightfrom the surface onto the photo-diode.

Preferably the lens means has an aperture which is selected so as tolimit the angular range of scattered light which will reach itsassociated photo-detector.

Preferably each of the monochromatic light sources is an LED.

It is preferable for the apparatus to include a pulsed power supply forthe LED's, such that the three LED's are operated alternately in series,with an off period to allow background light to be measured. Therepetition rate of the LED's preferably occurs in the range 1 KHz up to1 MHz, or beyond and is only limited by the time response of thephoto-detectors.

The apparatus may be electronically hard wired or may supply signals to,and be controlled by, a computer with a suitable interface and dataacquisition card. In the case of a general purpose computer, the chosenmethod is performed by suitably programming the computer to makemeasurements on and/or compute differences and/or ratios between, theoutput signal values to produce a classifying signal for a surface undertest.

The invention also lies in a method of identifying a material in or on asurface comprising performing any two or more of the different methodsas aforesaid in parallel, on output signals from photo-detectors inreceipt of light from the surface at two or more different angles, andgenerating a final classification or acceptance signal depending on theresults obtained from the two or more identifications, and in apparatusadapted to perform in parallel the two or more methods as aforesaid.

By the expression in parallel is meant simultaneously in time, or oneafter the other in quick succession with the results of the first methodbeing stored for combining with the results of the second and any othermethod to produce the final classification or acceptance signal.

Typically the interface or the apparatus includes analogue signalamplifying means and an analogue to digital converter for supplyingdigital signals to the data acquisition card, or the latter includes anA/D converter.

The invention is of particular use in the field of articleauthentication, such as checking passports, ID cards, driving licences,bank notes, bonds, share certificates, postage stamps, and othersecurity documents, although it is to be stressed that the invention isnot limited to this type of use, and the above are in any case onlyintended as examples of documents which can be checked.

In so far as inks, dyes and varnishes can be applied to any suitablesurface, the invention is not limited to the authentication ofdocuments, but can be used in connection with any article having asuitable surface to which an OVM can be applied.

In general the area of the coated or impregnated surface which is to beilluminated and from which reflected/scattered light is to be detected,needs to be flat. However the area concerned need only be relativelysmall, but if so the area on which the illuminating light is to fallneeds to be restricted so as to correspond to the size of the flat areawhich can be checked using reflected/scattered light therefrom.

It is worth noting that the time required for a test to be performed canbe very short if the light illuminating the surface can be turned on andoff very quickly and the detector can respond equally quickly. Thereforeprovided articles can be presented to and removed from an illuminatingand detecting device at high sped, and can be routed differently afterpresentation, depending on whether the light received by the detectorcauses an appropriate signal to be generated or not, the speed at whicharticles can be detected is more likely to be limited by the speed atwhich they can be moved into and out of the position at which the testcan be performed, rather than on the speed at which the test can beperformed by the device.

Although the ability to perform a test on a surface very quickly enablesthe invention to be incorporated into a process requiring a large numberof articles to be checked per minute, it is not limited to suchapplications, and it may be employed in a device to which articles aresupplied on an occasional basis, such as an off-line security documentverification/authentication device. The result of the test to establishwhether the article is genuine or not will be available almostinstantaneously, thereby involving little or no delay in the processingof articles which need to be verified/authenticated.

The invention may be employed in the field of quality control foron-line checking that particular inks, dyes or variables have beensatisfactorily applied to articles. Thus the invention is of applicationin the field of on-line checking of OVM coated sheet material before,during or after being printed or coated with other non-OVM material,depending on whether the OVM material is applied to the sheet materialbefore during or after the other non-OVM material is applied thereto.

The invention may thus be employed in the quality control of sheetmaterial which is to be printed to form security documents such as banknotes where OVM is applied to some or all of the surface of the sheetmaterial before it is printed to form the documents.

The invention may also be employed in the quality control of a processof printing sheet material in which OVM is applied to the surface of thesheet material during or after the printing process.

In the field of quality control of coated sheet material linear speedsof the sheet material of the order of 2.4 meters per second are typical.

Bank note sorting as between one denomination and another or separatingpossible forgeries from genuine notes using currently availableidentification techniques, is typically performed with linear speeds ofthe notes through the inspection station of the order of 10 meters persecond. However the speed of operation of apparatus operating inaccordance with the present invention, to sort notes incorporating OVMon the basis of OVM response criteria, will enable linear speeds ofnotes through the inspection station to be at least 20 meters persecond.

The invention therefore also lies in a method of checking an articlecontaining OVM in at least some of the surface to determine if thecorrect OVM has been employed, in which a surface of the article isilluminated and light scattered/reflected from the surface is detectedand an output signal generated in accordance with any of the previouslydescribed methods for detecting or determining an OVM in or on asurface, and in which an appropriate criterion is selected forgenerating an acceptance signal therefrom.

The invention also lies in a method of checking an article containing aparticular OVM to determine if the OVM is present in a particular regionof a surface of the article, in which the article is positioned relativeto a source of illumination and to detectors for receivingreflected/scattered light therefrom, with only the particular region ofthe surface being subjected to the illumination, and an output signal isgenerated in accordance with any of the previously described methods fordetecting or determining an OVM in or on a surface, and in which anappropriate criterion is selected for generating a final output signalindicating whether or not the particular OVM is present.

The invention also lies in a method of checking an article containingOVM in at least some of its surface to determine if the latter ispresent above some minimum concentration measured as quantity of OVM perunit area, in which the article is positioned so that the surface willbe illuminated and light reflected/scattered therefrom will be detectedand an output signal will be generated in accordance with any of thepreviously described methods for detecting or determining OVM content ofa surface, and in which an appropriate criterion is selected forgenerating a final signal indicating whether or not a particularconcentration of the OVM is present.

The invention also lies in checking apparatus for performing any of theaforesaid methods of checking an article.

In any of the foregoing methods or apparatus the sheet/substrate/articlecontaining the surface of interest may be static or moving during theillumination step, and may be undergoing printing or coating with theOVM and/or other materials, or may have been so printed or coated in aprevious process, or may comprise a finished article such as a securitydocument.

Checking apparatus as aforesaid may be fitted to or located downstreamfrom a coating or printing apparatus applying at least OVM to sheetmaterial passing therethrough to thereby enable on-line checking of OVMapplied to the sheet material by the coating or printing apparatus, tobe monitored.

Checking apparatus as aforesaid may be fitted to a document handlingapparatus to enable documents to be sorted according to the OVM found tobe present in or on a surface of each document.

Checking apparatus as aforesaid may be incorporated in a cash dispensingmachine, a banknote acceptor, or banknote-sorting machine in which thebanknotes incorporate OVM in or on their surface.

The invention will now be described by way of example with reference tothe accompanying drawings in which:

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an optical arrangement by which the reflectance ofdifferent inked/coated surfaces can be determined in relation to thewavelength of the incident light, for different incidence andobservation angles,

FIG. 2 is an explanatory diagram indicating scattering angle (θ_(s)),

FIG. 3 is a similar diagram indicating photo-detector angle (θ_(d)),

FIGS. 4A, 4B, 4C, 4D and 4E are graphical illustrations of experimentalresults obtained for the spectral properties of different sample inksusing the system of FIG. 1,

FIG. 5 is a layout diagram of an optical arrangement for determiningreflectance characteristics of a surface,

FIG. 6 is a layout diagram of an optical arrangement containing aviewing window, two LED light sources and two detectors for determiningthe reflectance characteristics of a printed/coated surface in theviewing window,

FIG. 7 is a schematic diagram of the control circuit for alternatelypowering the two LED sources of FIG. 6,

FIG. 8 is a schematic diagram of a classification circuit adapted toreceive analogue signals from the two detectors of FIG. 6,

FIG. 9 is a layout diagram of an optical arrangement containing aviewing window, two LED light sources and three detectors, fordetermining the reflectance characteristics of a printed/coated surfacein the viewing window, for use with and A–D converter and digital signalprocessing computer,

FIG. 10 is a schematic diagram of the control circuit for alternatelypowering the two LED sources of FIG. 9, using digital signals derivedfrom an A–D converter driven by the computer,

FIG. 11 is a schematic diagram of a signal processing circuit foramplifying analogue signals from the three detectors of FIG. 9, beforeconversion to digital signals for processing by the computer,

FIGS. 12, 13 and 14 are exemplary computer programs for controlling andhandling output signals from the detectors of FIGS. 9–11,

FIG. 15 is a layout diagram of an optical arrangement containing aviewing window, three LED light sources and three detectors, fordetermining the reflectance characteristics of a printed/coated surfacein the viewing window, for use with an analogue acquisition card with acomputer used for digital signal processing.

FIG. 16 is a schematic diagram of the control circuit for alternatelypowering the three LED sources of FIG. 9, using digital signals derivedfrom an analogue acquisition card driven by the computer.

FIG. 17 is a schematic diagram of the amplifier circuit for thephotodiodes prior to acquisition by a computer driven analogueacquisition card.

FIG. 18 is a flow chart of a computer program for implementing a firstmethod (method I) to be described,

FIGS. 19( a), (b) and (c) are graphical plots of log (hue) ratios fromdifferent pairs of detectors in the arrangement of FIG. 9—used to derivea logic table for use in a second method (method II) to be described,

FIG. 20 is graphical illustration of hue ratio relationships withscattering angle for different ink types—also used to derive a logictable for use in method II,

FIG. 21 is a command and logic diagram of a program for controlling acomputer to implement method II, and

FIG. 22 is a command and logic diagram for controlling a computer toimplement a third method (method III) to be described.

DETAILS OF TABLES 1, 2 AND 3 SET OUT AT THE END OF THIS DESCRIPTION

Table 1 is a listing of the different hues observed at different viewingangles, for each of twelve sample inks (numbers 1–12 in the Table).

Table 2 indicates the optimal detector angles using 45° incident lightand the optimal detection wavelengths of the twelve sample inks of Table1, and additionally classifies a sample as OVM, pearlescent OVM ormetallic.

Table 3 lists the hue ratios measured for several sample materials by anarrangement based on FIGS. 9 & 10 using two illumination wavelengths at654 nm and 574 nm, and three photo detector scattering angles of 45, 90and 110 degrees.

Table 1 shows details of the visual properties of commercially availableOVM inks. These inks generally fall into two categories, non pearlescentand pearlescent. In Table 1 inks 1, 2, 3 and 12 are examples ofnon-pearlescent OVMs, inks 5 to 11 are pearlescent and ink 4 is ametallic ink which is not an OVM, and is thus viewed as the same colourregardless of viewing angle.

DETAILED DESCRIPTION OF THE DRAWINGS

The optical reflectance characteristics of a surface coated with an inkcan be determined as a function of wavelength using a standardcommercially available spectrometer set up as shown in FIG. 1.

Thus in FIG. 1 light from a broad band (white light) lamp 10 isprojected through a slit 12 onto a plane mirror 14 from where it isreflected onto a first convex mirror 16. Light from 16 is divertedtowards a reflection grating 18 after which a similar path via a secondconvex mirror 20 and plane mirror 22 leads to an exit slit 24 at thefocal point of the second convex lens (taking into account the pathchange introduced by mirror 22).

After passing through the exit slit 24 the now diverging light beam iscollimated by a convex lens 26 and a parallel beam of monochromaticlight 28 is projected onto the inclined surface of a test sample 30.

Light reflected by the latter is focused by an imaging lens 32 onto aphotodiode 24 the output of which is amplified by amplifier 36 toprovide an output signal at 38.

By locating the convex mirrors 16 and 20 equidistant from the slits 12and 24 and the grating 18, respectively, so the mirror 16 forms an imageof the slit 12 on the grating, and the mirror 20 forms an image of theslit image reflected by the grating at the slit 24.

Altering the angle of the grating 18 alters the wavelength of the lightin the reflected beam travelling towards the prism 20, and items 12 to24 comprise a known Hilger-Watts monochromator.

Tests on a particular sample 30 involve rotating the grating 18 andobserving how the signal at 38 changes. The variation of signal can beplotted relative to wavelength (which is proportional to the tilt angleof 18).

A second test can be performed by selecting particular wavelengths (byappropriate adjustment of the tilt angle of 18) and/or moving theposition of the lens and photodiode combination 32, 34 relative tosample 30, so as to view the surface of the sample at different angles,for each wavelength of the illuminating light. In these tests the angleof illumination remains constant.

A third test involves performing Test 2 with the sample 30 set on otherangular orientations relative to the axis of the beam 28 and alteringthe angle of the sample 30 relative to the axis of beam 28 while makingadjustments to the position of the lens/diode simultaneously alteringthe position of the diode combination 32, 24 to maintain a constantincluded angle between the beam 28 and the optical axis of line 32.

In Test 2 and Test 3 the variation of the signal at 38 can be observed(and plotted if desired) vis a vis the viewing (or observing) angle inTest 2, and illumination/viewing angle variation in Test 3.

A set of curves can be obtained by varying the wavelength of the lightimpinging on the sample 30 for each of the different angles in eachtest.

FIGS. 4( a) and 4(b) illustrate a set of curves obtained using a surfacecoated/printed with ink sample 2 of Table 1. FIG. 4( a) corresponds toTest 2 and FIG. 4( b) corresponds to Test 3 above. In each case thevariations of signal at 38 against wavelength is plotted for each of anumber of different angles θ_(s) (15°–120°) in FIG. 2( a) and θ_(d)(15°–70°) in FIG. 2( b).

Again using the system of FIG. 1 optical reflectance characteristicshave been determined for all 12 inks of Table 1, for two differentviewing and illumination configurations as follows:

-   (i) Collimated illumination is incident upon the ink at a pre-set    angle of 45° to the normal, and the reflectance spectrum is recorded    as a function of the scattering angle θ_(s)—that is the angle    between the illumination source and the photo-detector as defined in    FIG. 2( a).-   (ii) Scattering angle is fixed at 90° and the reflectance spectrum    is measured by varying just the angle θ_(d) of the photo-detector    with respect to the paper, as defined in FIG. 2( b).

The following properties have been observed for the respective inktypes:

-   1. Non-pearlescent OVM inks (Samples 1, 2, 3 & 12)—these inks    exhibit a definite reflectance peak with a half width of about 130    nm, which shifts towards the blue end of the spectrum with    increasing scattering angle. This is shown in the upper graph of    FIG. 4C for ink Sample 1. When the scattering angle is held constant    only slight shifts in the peak are observed with detector angle, as    shown in the lower graph of FIG. 4C for ink Sample 1.-   2. Pearlescent OVM inks (Samples 5 to 11)—the reflectance spectrum    of these inks are relatively flat for almost all illuminating and    viewing angles, hence appearing “almost” white. The exception to    this is when direct specular reflection is viewed (θ_(d)=θ_(s)=90°)    at which a perturbation over a broad wavelength range is observed    giving rise to the observed increase in colour. This is shown in    FIG. 4D for Sample ink 10.-   3. Non OVM metallic ink (Sample 4)—very little change in the    spectrum is observed with scattering and detector angles, as shown    in FIG. 4E.

From the acquired data, the optimal wavelengths and angles for detectionfor the twelve sample inks have been determined and are shown in Table2.

First Detector and OVM Detection Method

FIG. 5 shows one detector arrangement by which up to three outputsignals at up to three different detector angles can be obtained usingwhite light to illuminate a sample. As shown a white light source 40projects light via a mirror 42 and imaging optics (lens assembly) 44onto a sample surface 46. Reflected/scattered light is viewed at threedifferent angles to the surface by three photodiodes located in housings48, 50 and 52 each containing an imaging lens 54, 56, 58 and aphotodiode 60, 62, 64 respectively.

If the surface 46 is plain white (or a matt ink of uniform colour) thelight incident on each of the photodiodes 60, 62 and 64 will be ofsubstantially the same frequency spectrum. However if the surfacecontains OVM the wavelength of light directed towards 60 may bedifferent from that directed towards 62 and that may be different againfrom that directed towards 64.

In order to determine if an OVM is present, it is necessary to determinewhether the light reflected along the different directions is of aparticular wavelength. Depending on the criterion based on Tests 1, 2 or3 above or the algorithm selected to process the values of outputsignals from the photo-detectors (see methods I, II and III later) sothe outputs from either two or all three of the photo-detectors arerequired. To cover both possibilities three filters are provided at 66,68 and 70 each being a band pass filter restricting the unattenuatedlight to a specific wavelength or narrow band of wavelengths.

Alternatively or additionally similar filters (not shown) may beemployed between the source 40 and mirror 42 or between the latter andthe lens assembly 44 to restrict the light incident on the surface atany one time, to one of a plurality of different wavelengths.

A plurality of filters may be employed in groups of two or three anddifferent groups selected to produce and/or look for differentwavelengths of light both towards the surface and/or along the two orthree reflection paths from the surface required for any particularalgorithm.

A disadvantage of this arrangement is that much of the light from thesource 40 is unavailable for detection by the photodiodes, sincebroadband white light is employed. However the arrangement does notrequire monochromatic light sources to be employed.

Alternative OVM Detectors and Methods of Detecting OVM

The following alternative arrangements require monochromatic lightsources.

The following alternative detectors have been developed to identify andquantify the presence of an OVM on a surface, and some are adapted to becontrolled by and to supply output signals to a commercially availablecomputer fitted with a data acquisition card.

In FIG. 6, 2 LEDs 80, 82 and two photodiodes 84, 86 are arranged in alight-tight housing 88 relative to a window 90 through which the surfaceof a sheet of material 92 stretched across or moving below the undersideof the housing 88, can be illuminated (by the LEDs) and viewed (by thephotodiodes). The light from the LEDs is directed towards the window andeach of the diodes 84, 86 includes a lens 92, 94 and receives lightreflected from the surface below the window, at 60° and 120°respectively.

The LEDs are powered alternately from a switched power supply such asshown in FIG. 7. The light output from each LED and the balance betweenthem can be adjusted by altering the value of the variable resistors 96,98. Timer 100 produces switching pulses at 5 KHz to trigger first oneand then the other of two semiconductor devices 102, 104 and the logicdevice 106 ensures that if 102 is conducting 104 is non-conducting andvice versa.

The switching speed can be set at any frequency governed by the responseof the photodiodes 84, 86. Ideally the switching is set to occur at ahigh frequency such as 1 MHz and the components 84, 86, 100, 104 and 106would be chosen accordingly.

The detectors 84, 86 provide input signals to an analogue amplifying andclassification circuit shown in FIG. 8. Here the signals from bothdiodes are amplified using the different inputs of an operationalamplifier 108, but the signals from detector 84 are subjected topre-amplification by an operational amplifier 110 the gain of which isadjustable by altering the value of the feedback resistor 112.

The circuits of FIGS. 7 and 8 are set up using a sheet of white paper at92. This gives rise to a dc signal from both 84, 86 and the value of 112is adjusted to produce a zero output signal in the output of the thirdoperational amplifier 114, with the white sheet in place.

Variable resistor 116 determines the threshold at which operationalamplifier 114 changes state to produce a current for driving indicatingLED 118, and the value of 116 is adjusted by replacing the white paperwith a flat surface containing the OVM ink to which the device is torespond in use, and adjusting 116 until the LED illuminates.

Tests on a device constructed in accordance with FIGS. 6 to 8 usingHewlett-Packard HLMP-C515 and HLMP-215 LEDs and Texas Instruments TSL251lensed photodiodes as items 82, 84 indicated that it could distinguishbetween OVM and non-OVM (and pearlescent) inks without error for inksamples 1, 3 and 12 of Table 1, and produced a “grey scale” outputrepresenting the concentration or quality of the OVM ink. The switchingspeed was governed by the response of the selected diodes 84, 86 but byselecting different photodiodes, this could be increased from 5 KHz inthe circuits shown, to 1 MHz if desired.

An alternative device is shown in FIG. 9. This is essentially the sameas the arrangement shown in FIG. 6 with the addition of a thirdphotodiode 120 located midway between the other two photodiodes to allowsignals at 90° from the incoming light from the LEDs to be interrogatedby the third photodiode 120. The other items in the arrangement shown inFIG. 9 are identified by the same reference numerals as used in relationto FIG. 6 and the functionality of the devices is as described inrelation to the earlier Figure.

The LEDs 82 and 84 in FIG. 9 may be driven by a switched power supplysimilar to that shown in FIG. 7 or the power supply shown in FIG. 10 inwhich the pulse generating circuit based on the 555 semiconductor device100 is removed and pulses from a data acquisition card linked to acomputer which operates to process the output signals, or from ananalogue to digital converter, are supplied in place of the pulses fromthe device 100. Again the components in the remainder of the circuit ofFIG. 10 are the same as those in FIG. 7 and the same reference numeralsare used to denote the items concerned.

No detail is shown of the analogue to digital converter since this is aproprietary item and the preferred device is a Pico ADC-11, 10 channel,analogue to digital converter which can be obtained from RS Componentsunder Part No. 830053.

A personal computer (not shown) can be used to control the switching ofthe two LEDs via the ADC.

The voltage outputs from the three photodiodes 84, 86 and 120 provideinputs to three operational amplifiers 122, 124 and 126 shown in FIG.11. Each includes a resistive feedback loop so that the gain of each canbe adjusted to compensate for different output signal levels from thediodes. The outputs appear on lines 128, 130 and 132 which supply threeinputs of the ADC-11 analogue to digital converter. The latter serves todigitise the analogue signals on lines 128, 130 and 132 to providedigital information to a computer (not shown) to enable the signals tobe processed and analysed digitally, as well as providing synchronousLED switching signals for the LED control circuit of FIG. 10.

Three computer programs are set out in FIGS. 12, 13 and 14, each ofwhich if run on a personal computer will analyse the digital data fromthe outputs 128, 130 and 132 to provide a classification of an inkprinted or coated surface in the window 90.

The program shown in FIG. 12 only takes the data from lines 128 and 132of FIG. 11 and ignores the 90° photodiode 120. The logic and processingreplicates the operation of the circuit shown in FIG. 8 and provides anoutput which indicates that an OVM has been detected if an output signalis greater than some threshold.

The second program of FIG. 13 utilises all three photodiode signals andgenerates red/green ratios at each of three scattering angles.

Using this computer-based system, the sensor was able to identifysamples 1, 2 and 3, 5, 7 and 11 from Table 1, for which the LED outputof wavelengths were optimal, and misclassification was negligible.

The program of FIG. 13 allows a non-linear nearest neighbour classifierto be constructed which is optimal in the sense that it will identifythe exact parameters that characterise a given ink. The resulting systemis very ink specific and classifies an ink according to a given inkbeing present in exact quantities. Insofar as the device is to be usedas a quality control device, this is no serious limitation and aclassifier constructed using the computer program shown in FIG. 13 willprovide just the sort of tool required to maintain high consistentquality of OVM printed surfaces.

The third program, of FIG. 14 broadly classifies a given ink asisotropic, pearlescent, OVM or non-pearlescent OVM. It can be seen froman analysis of the measure R-G ratios shown in Table 3 that printed inks(both OVM and non-OVM) fall into four general categories. The first twocategories cover matt and glossy inks, and the latter two categories OVMand pearlescent inks. Matt inks appear to exhibit constant red-greenratios with scattering angle, whereas glossy inks exhibit ade-saturation of colour (that is the R-G ratio tends to unity) forspecular reflection. On the other hand, OVM inks for which the sensor isoptimised, exhibit a decrease in the R-G ratio with increased scatteringangle, and these are always substantial. Finally the pearlescent inksfor which the sensor is preferably optimised (namely for samples 5, 7and 11) exhibit a saturation in colour, that is a divergence from unityof the R-G ratio, for specular reflection.

The non-linear classification strategy of the third computer program inFIG. 14 allows a broader classification according to ink type and anestimate of the quantity of ink present on the white substrate. Thisallows a broad classification of ink type and the best classificationstrategy is to be found by dividing the three-dimensional spaceconstructed from the logarithm of the red-green signals at each of thescattering angles, into regions. In this way inks can be broadlyclassified as to type, and the distance from the centre of the region(which corresponds to white paper), provides an estimate of the amountof each ink present. This concept will be described later in more detailin relation to FIG. 19.

The very accurate ink identification characteristics of the secondprogram (FIG. 13) and the broad classification characteristics of thethird program (FIG. 14) mean that an overall classifier may require twochannels, one operating the FIG. 13 program, and the other the FIG. 14program, to allow overall classification and ink identification to beperformed quickly and accurately.

In FIG. 15 the photodiodes and LEDs are housed within a housing 140. Redand Green LEDs 142, 144 are mounted within a block 148 and a third BlueLED 146 is mounted within an adjacent block 150, to facilitate themounting of the LEDs within the casing. The Red and Green axes areperpendicular and the axis of the Blue LED is parallel to the Red axis.Dichroic mirrors 152, 154 at 45° to the LED axes combine the light fromthe three LEDs into a single axis 158 (coaxial with the Green axis inthe arrangement shown) and a sub assembly is formed using the two blocks150, 152 and a thin walled box, two walls of which are denoted by 154,156 which co-operate with the blocks and other side walls of the box toform an enclosure which traps all the light from the LEDs within theenclosure except light along the axis 158 which can pass out through anaperture 160 in the wall 154.

Escaping light is reflected by a mirror 162 mounted within the casing soa to project light through an imaging lens assembly 164 which can passout of the casing through a window 166 in the underside thereof.

A platform 168 associated with the casing 140 and spaced from the window166 by a predetermined distance serves as a support for a substrate 170which carries in or on its upper surface a coating of an ink, dye orother material which may contain an OVM.

Three photo-detectors 172, 174, 176 are located within three housings178, 180, 182 respectively which also house focusing lenses 184, 186 and188 for focusing light reflected by the surface of the substrate 170, at45°, 90° and 110° relative to the direction of the incident light 190from the imaging lens assembly 164, along axes 192, 194 and 196.

The lens assembly 164 and the distance from the window 166 to thesubstrate surface 170 (typically 20 mm) are selected so that a smallspot of light some 2 mm in diameter is formed on the substrate surface.The imaging lens assembly 164 preferably has a 5 mm aperture limitingthe angular range of the illumination incident on the surface to ±10degrees about the axis 190.

In general the angle between axis 190 and the normal to the substratesurface is selected to be 45°. This angle is expressed in this way,since the surface may not be entirely flat and may even be curved, butprovided the region on which light from 164 is incident is flat over anarea at least 2 mm in diameter, and the axis 190 is at 45° to the normalto that region of the surface, it is relatively unimportant if theremainder of the substrate is not coplanar therewith.

If checking the material in or on the same part of each of a pluralityof similar substrates (e.g. similarly sized security documents such asbanknotes) it is merely necessary for each document to replace theprevious one in the same position relative to the window 166 on theplatform 168.

If checking that a continuous length of sheet material has a requiredmaterial in or on its surface as a result of a printing and/or coatingtreatment, it is merely necessary to move the sheet material relative tothe window, whilst maintaining the region illuminated by the light from164 at a constant distance from the window and in the correct plane.

In either event the three LEDs may be operated in rapid succession sothat the substrate is successively illuminated by Red, Green and Bluelight and the photodiodes are addressed at appropriate points in timecorresponding to the three illumination intervals, so that the substratesurface and the three photodiodes are only presented with light of onewavelength during each illumination interval.

Where the substrate is replaced by another to be illuminated andevaluated in the same way, the LEDs and/or the photodiodes may beinhibited during the replacement.

FIG. 16 shows how three LEDs 146, 148 150 are sequentially switched ONand OFF by current from pins 2, 3 and 4 of device 198 (a type 74LS145counter). Pin 15 of 198 is connected to pin 3 (the Q_(o) output) ofdevice 200 (a type 74LS193 counter) pin 2 of which is connected to pin14 of 198 and pin 1 of device 202 (a type 74LS123 device).

A 100 KHz signal is supplied from a data acquisition card type DT7102(not shown) via an amplifier 203 to pin 5 of device 200. Pin 13 of 200provides signals to the card DT7102 to control the reading of the outputsignals of the photodiodes 172, 174 and 176.

Successive illumination of the LEDs 146, 148 and 150 and the synchronousinterrogation of the photodiodes 172, 174 and 176 is controlled by clockpulses derived from the 100 KHz signal. Since each transition of the 100KHz signal can be used to generate a clock pulse, the latter will begenerated at the rate of 200,000 per second. The illumination andinterrogation process is performed by a succession of 16 clock pulses.

The first of the 16, pulse 1 causes pin 1 of 198 to go LOW and remainLOW for pulses 2–4. Pulses 2, 3 and 4 cause the outputs of photodiodes172, 174 and 176 to be read and stored. Since pins 2, 3 and 4 of 198remain HIGH during this time, all three LEDs remain off and the outputsfrom the photodiodes during pulses 2, 3 and 4 will correspond tobackground light reflected by the surface of 170 and received by each ofthe photodiodes.

The fifth of the 16 pulses causes pin 1 of 198 to go HIGH and pin 2 togo LOW and remain LOW for pulses 6, 7 and 8. As with pulses 2, 3 and 4,pulses 6, 7 and 8 cause photodiodes 172, 174 and 176 to be read insuccession. Since pin 2 of 198 is LOW, the Red LED 146 will beilluminated, and during pulses 6–8 the output signals from 172, 174, 176will correspond to Red light reflected from the surface of 170.

With the arrival of pulse 9, the LOW output of 198 transfers to pin 3causing the Green LED 148 to illuminate, and during pulses 10, 11 and 12the response of the photodiodes to reflected Green light from thesurface 170 is read out.

With the arrival of pulse 13, the LOW output of 198 transfers to pin 4,causing the Blue LED 150 to illuminate and during pulses 14, 15 and 16the response of the photo-detectors to reflected Blue light from thesurface 170 is read out.

The process will repeat during each consecutive group of 16 clockpulses.

It has been found advantageous for pulses 1, 5, 9 and 13 to be redundantas far as read-out of the photodiodes is concerned, since although theLEDs switch ON and OFF instantaneously, there is a short rise timeassociated with photodiode operation and the short period of timebetween the arrival of pulses 5 and 6 (9 and 10, and 13 and 14), givesthe photodiodes a chance to respond to the new light level incidentthereon as each of the LEDs is turned on in turn. In this way thephotodiodes will be in a steady-state condition when their outputsignals are read out.

FIG. 17 shows one channel of a three channel signal amplifier foramplifying and shaping the signals from the photodiodes 172, 174 and176. Two MC33274 quad op-amplifier devices 204, 206 are employed, onechannel of each being left unused.

Corresponding amplifiers in each of 204, and 206 are connected in serieswith feedback via resistor networks 208, 210 and 212, 214 being appliedto the inverting input signal of each amplifier. Adjustment of thevalues allows the signal gain of each channel to be varied, ifnecessary, to balance up the output signal levels from the photodiodes.An input resistor 216 and shunt capacitor 218 connects each photodiodeto its respective amplifier input. Capacitor 218 is typically InF.

The amplified photodiode output signals are applied to three inputs ofthe multi-channel A/D convertr of the data acquisition board (not shown)type DT7102.

Suitable photodiodes are TSL 252.

The LEDs are selected to produce monochromatic light of 654 mm (Red),574 mm (Green) and 472 mm (Blue). As described above they areflash-illuminated repetitively in a cycle.

The repetition rate of the cycle is 12.5 KHz if a clockpulse rate of 200KHz is employed. This is software controlled by a computer via a dataacquisition card producing a 100 kHz signal. The LEDs are chosen to giveas good a match for as many optimal measurement wavelengths (as in Table2) as possible.

Light scattered and reflected by the surface of 170 is collectedsimultaneously by the three photodiode units 178, 180 and 182 spaced atscattering angles of 45°, 90° and 110° from the LEDs, each of which isfocused on the illuminated portion of the surface. The scattering anglesof the photodiodes are chosen to be as close as possible to the optimaldetector angles given above, given also the practical considerations oftheir physical size and mounting. The amplified voltages generated fromthe three photodiodes are converted to digital values using anacquisition card (not shown) after which the three output signal valuescan be stored in a computer (not shown).

The computer is then used to process the recorded data to determinewhether a particular material is present in or on the surface, andwhether or not it is in the required quantity. This is done using one ofthree programs, each subjecting the photo-detector output signal valuesto one of three different algorithms, thereby performing one of threedifferent methods, detailed as follows:

Method I

This gives a simple linear measure of the quantity of a non-pearlescentOVM present on a substrate using just two LEDs and two of thephotodiodes at scattering angles of 45° and 110°. The two LEDs used arechosen according to the properties of the OVM to be looked for, asdetailed in Table 2. The method can only determine if the one materialtype is present on the substrate, and may be confounded if two or moreOVMs are present. Using just the red and green LEDs to detect, say, inkSample 1, the technique proceeds by calculating the value of M using thealgorithm:M=|(R ₁₁₀ −R ₄₅)−A*(G ₁₁₀ −G ₄₅)|

In a first calibration step, the photodiode amplifier gains are adjustedto compensate for any alignment differences, so that R₁₁₀=R₄₅.

The constant A can then be set. A is a constant that is adjusted tocompensate for differences between the responses of the two photodiodesto the illumination wavelength. It is found by illuminating matt whitepaper, and adjusting the value of A until M is reduced to zero.

When a substrate printed with a non-pearlescent OVM ink is illuminatedinstead there will be a difference between the values of (R₁₁₀−R₄₅) andA*(G₁₁₀−G₄₅) giving rise to a non zero value for M. The magnitude of Mfor any given ink relates to the quantity of the ink on or in thesubstrate. Matt substrates (e.g. white paper, coloured card) do notproduce these differences, giving rise instead to R₁₁₀ being very closeto R₄₅ and G₁₁₀ being very close to G₄₅, thus resulting in a zero ornear zero value, for M.

If the value of M is sufficiently greater than zero to indicate that thenon-pearlescent OVM ink is present, the actual value of M can becompared with a threshold value t (found by experiment using differentconcentrations of the OVM concerned and observing values of M). Alook-up table of values of M can be assembled from such data if desired,to provide indications of concentration in relation to measured valuesof M.

If the value of M is less than a particular value (i.e. is too close tozero) it is not necessarily true to assume that this corresponds to avery low concentration of the particular OVM, since this may indicatethat no OVM is present and the actual value of M is merely due toerrors, resulting in a non-zero value of M, when it should actually bezero.

A flow chart of the logic and decisions to be performed by a computer todetermine whether a particular OVM is present is set out in FIG. 18.

This particular method has been described earlier, in relation to FIG.6, where it has been shown that A/D conversion is not necessary, andsimple analogue techniques may be employed to determine if thephoto-detector output signals indicate if a particular OVM is present.The foregoing illustrates how a general purpose detector incorporatingmore LEDs and photodiodes than are actually necessary to perform MethodI, using a Data acquisition card with A/D conversion and a computer, canalso be used to perform the same method.

Method II

This method identifies OVM inks by examining the relationship of theratio of one colour with respect to another reflected by a particularOVM ink, as a function of increasing amount of ink printed on orotherwise applied to a white substrate. It is also useful fordiscriminating between specific ink group types irrespective of thequantity of ink printed.

Although many combinations of LED illumination wavelengths are possible,measurements using just red and green LEDs but inspecting reflectedlight at three different angles, allows many of the samples in Table 2to be differentiated between, using just two LEDs and three photodiodes.

Samples 8, 9 and 10 cannot be discriminated between using just red andgreen LEDs, but a similar technique utilising either red and blue, orblue and green LED combinations, allows these inks to be identified.

FIG. 19 shows graphs of logarithms computed for the red-green ratios (orhue values) themselves calculated for a wide variety of different inksamples detailed in Table 2. The red-green ratios (or hue values)h_(Ø1), h_(Ø2), h_(Ø3) are calculated after removing background light(measurements D₄₅, D₉₀ & D₁₁₀), as follows:h _(Ø1) =h ₄₅=(R ₄₅ −D ₄₅)/(G ₄₅ −D ₄₅)h _(Ø2) =h ₉₀=(R ₉₀ −D ₉₀)/(G ₉₀ −D ₉₀)h _(Ø3) =h ₁₁₀=(R ₁₁₀ −D ₁₁₀)/(G ₁₁₀ −D ₁₁₀)

FIG. 20 is a schematic representation of the decision matrix of FIG. 19.This can be best uderstood by considering a system set up so thatreflected Red light gives small h_(Ø1) h_(Ø2) and h_(Ø3) whereasreflected Green light gives larger values of h_(Ø1) h_(Ø2) and h_(Ø3).

For a non-pearlescent OVM, light leaving the surface at a low scatterangle will tend to look green, and will thus produce a high h_(Ø1) valuewhereas light at a larger scattering angle will tend to look Red andproduce a low h_(Ø3).

Thus for such an OVM, h_(Ø1)>h_(Ø3).

Specular reflection (somewhere between the two scattering anglesconsidered above) will tend to be more Red than Green, so that h_(Ø2)will normally be between h_(Ø1) and h_(Ø3).

For a pearlescent OVM, light leaving the surface at large and smallangles of scatter will tend to white, thus h_(Ø1) and h_(Ø3) will bothtend to unity. However light due to specular reflection will posses morecolour and h_(Ø2) will diverge from unity.

For example if the light reflected at high and low scatter angles is avery pale red (i.e. more white than red) the specular reflection willlook very red and h_(Ø2) will be a small value<1.

Therefore in this case:h_(Ø2)<h_(Ø1) and h_(Ø2)<h_(Ø3),where h_(Ø1) and h_(Ø3) both tend to be just less than unity.

In an alternative example where the reflected light at the high and lowscatter angles is a very pale green (i.e. more white than green), h_(Ø1)and h_(Ø2) are again very close to unity (albeit this time slightlygreater than unity), and the specular reflection will look very green,and the value of h_(Ø2) will be large—and certainly larger than h_(Ø1)and h_(Ø3).

Therefore in this case:h_(Ø2)>h_(Ø1) and h_(Ø2)>h_(Ø3),

For a glossy ink, let us consider a system set up so that reflected Redlight give small values for h_(Ø1), h_(Ø2) and h_(Ø3) whereas reflectedGreen light gives high values for h_(Ø1), h_(Ø2) and h_(Ø3).

Here a glossy Red ink will yield small values for h_(Ø1) and h_(Ø3) atlarge and small scatter angles whereas specular reflection will tend tolook white, so that h_(Ø2) will be larger than both h_(Ø1) and h_(Ø3).

Conversely if the ink is a glossy Green ink, then it will look green athigh and low scatter angles, so yielding larger values for h_(Ø1) andh_(Ø3) and specular reflection will tend to look white, so that h_(Ø2)will be smaller than both of h_(Ø1) and h_(Ø3).

A flow chart of the logic and decisions required to be performed by acomputer to discriminate between non-pearlescent OVM, pearlescent OVM,and other inks is shown in FIG. 21.

A measure, Q, of the quantity of ink (i.e. concentration) in or on thewhite substrate, can be calculated using the values of R₄₅, G₄₅, D₄₅etc. as follows:

$Q = {{( {{\ln( {\lbrack {R_{45} - D_{45}} \rbrack/\lbrack {G_{45} - D_{45}} \rbrack} )} - 1} )\hat{}2} + \mspace{50mu}{( {{\ln( {\lbrack {R_{90} - D_{90}} \rbrack/\lbrack {G_{90} - D_{90}} \rbrack} )} - 1} )\hat{}2} + \mspace{50mu}{( {{\ln( {\lbrack {R_{110} - D_{110}} \rbrack/\lbrack {G_{110} - D_{110}} \rbrack} )} - 1} )\hat{}2}}$Method III

This method is suitable for characterising and discriminating betweenseveral individual OVM inks provided the LED output wavelengths areselected for the particular individual inks. In fact the technique isnot restricted to OVM inks and works equally as well with matt andglossy inks. It uses nearest neighbour classification, and may beconfounded if the ink to be detected has not been printed in sufficientquantity. This latter problem can be overcome, at least in part byperforming two or more classifying calculations in parallel fordifferent ink quantities. Likewise alternative ink types can be detectedmore reliably by performing additional classifying calculations inparallel.

FIG. 22 lists the logic steps and decisions needed of a computer toclassify one OVM ink printed at a particular density.

First the relative hues for each scatter angle are calculated from therecorded data after removing background light:r ₄₅=(R ₄₅ −D ₄₅)/(R ₄₅ +G ₄₅ +B ₄₅−3*D ₄₅)g ₄₅=(G ₄₅ −D ₄₅)/(R ₄₅ +G ₄₅ +B ₄₅−3*D ₄₅)b ₄₅=(B ₄₅ −D ₄₅)/(R ₄₅ +G ₄₅ +B ₄₅−3*D ₄₅)r ₉₀=(R ₉₀ −D ₉₀)/(R ₉₀ +G ₉₀ +B ₉₀−3*D ₉₀)g ₉₀=(G ₉₀ −D ₉₀)/(R ₉₀ +G ₉₀ +B ₉₀−3*D ₉₀)b ₉₀=(B ₉₀ −D ₉₀)/(R ₉₀ +G ₉₀ +B ₉₀−3*D ₉₀)r ₁₁₀=(R ₁₁₀ −D ₁₁₀)/(R ₁₁₀ +G ₁₁₀ +B ₁₁₀−3*D ₁₁₀)g ₁₁₀=(G ₁₁₀ −D ₁₁₀)/(R ₁₁₀ +G ₁₁₀ +B ₁₁₀−3*D ₁₁₀)b ₁₁₀=(B ₁₁₀ −D ₁₁₀)/(R ₁₁₀ +G ₁₁₀ +B ₁₁₀−3*D ₁₁₀)

These are then compared with relative hues from a pre-measured specimensample of the ink identical to that desired to be detected. Assume thatthe pre-recorded hues of the specimen ink are denoted as α₄₅, β₄₅, γ₄₅,α₉₀, β₉₀, γ₉₀, α₁₁₀, β₁₁₀, γ₁₁₀ for the hues of r₄₅, g₄₅, b₄₅, r₉₀, g₉₀,b₉₀, r₁₁₀, g₁₁₀ and b₁₁₀ respectively. The comparison is achieved usinga nearest neighbour classifier according to vector distance mathematics.Put simply the value of N is computed using the following equation:

N = (r₄₅ − α₄₅)² + (g₄₅ − β₄₅)² + (b₄₅ − γ₄₅)²+    (r₉₀ − α₉₀)² + (g₉₀ − β₉₀)² + (b₉₀ − γ₉₀)²+    (r₁₁₀ − α₁₁₀)² + (g₁₁₀ − β₁₁₀)² + (b₁₁₀ − γ₁₁₀)²

If the calculated value of N is below a certain threshold L, that isN<L, then it can be said that the desired OVM ink has been detected andexists on the substrate in the correct quantity (density). Different inkquantities, or qualities, or types can be looked for simultaneouslyusing different data sets of values for α₄₅, β₄₅, γ₄₅, α₉₀, β₉₀, γ₉₀,α₁₁₀, β₁₁₀ and γ₁₁₀ pre-measured from appropriate specimens.

The methods and apparatus described herein may be employed for articleauthentication, quality control of sheet material having and OVM appliedto its surface, and to document sorting.

Thus a method of article authentication wherein genuine articles arecoated over at least part of their surface with a known OVM comprisesthe step of illuminating an article in accordance with any of themethods claimed herein and selecting the scattering and photo-detectorangles and the wavelengths of the illuminating light in accordance withthe OVM which should be on the article, and comparing the output signalvalue produced by the method against a look-up table (having at leastone value therein) to generate an authentication or rejection signaldepending on the value of the output signal.

Such a method can be used to authenticate a passport, and ID card, adriving licence, a banknote, a bond, a share certificate, a postagestamp, or any other security document.

Thus a method of checking the quality of the printing or coating of atleast part of a substrate surface so as to deposit a particular OVMthereon comprises the step of illuminating the part of the surface whichhas been printed or coated with the OVM and investigating the reflectedlight in accordance with a method as claimed in any of claims 18 to 103and generating a pass or fail signal depending on the value of theoutput signal generated by the method.

The printed substrate can be sheet material which has been or is being,or will be printed, to form one or more security documents.

The security documents may be banknotes in which the OVM is applied tosome or all of the surface of the sheet material before during or afterit is printed to create the banknotes.

Apparatus adapted to print or coat a specific OVM to sheet material maybe combined with apparatus as described herein adapted to generate apass or fail signal depending on the value of the output signal producedby the apparatus as printed or coated sheet material moves relativethereto.

Document sorting apparatus for sorting documents according to whether aparticular OVM is in or on the surface of the document may be combinedwith apparatus as described herein adapted to generate a first controlsignal if the value of the output signal generated by the apparatus fora specific document indicates the particular OVM is present thereon anda second control signal if the output signal indicates no OVM to bepresent, and supplying a route controlling signal to the routeingapparatus to route the specific document to one of two destinationsdepending on whether a first or second control signal is generated bythe apparatus.

TABLE 1 General visual properties of sample inks. Hue Observed atVarious Viewing Angles Sample Number Back Scatter Specular Glancing 1Purple Orange Green 2 Orange/Pink Green Blue 3 Purple Orange Green 4Gold Gold Gold 5 White Light Green White 6 White Pink White 7 SilverGold Silver 8 White Light Violet White 9 Light Green Light Red LightGreen 10 Light Yellow Light Violet Light Yellow 11 Light Pink AppleGreen Light Pink 12 Purple Orange/Green Green

TABLE 2 Optimal detector angles for 45° incident beam and OptimalDetection Sample Number Ink Type Angles θ₅ Best Wavelength 1 OV 0°, 135°550 nm, 650 nm 2 OV 0°, 135° 520 nm, 600 nm 3 OV 0°, 135° 560 nm, 660 nm4 (Metallic) — — 5 Pearlescent 0°/135° & 90° 500 nm, 700 nm 6Pearlescent 0°/135° & 90° 525 nm, 640 nm 7 Pearlescent 0°/135° & 90° 525nm, 650 nm 8 Pearlescent 0°/135° & 90° 460 nm, 600 nm 9 Pearlescent0°/135° & 90° 510 nm, 700 nm 10 Pearlescent 0°/135° & 90° 460 nm, 580 nm11 Pearlescent 0°/135° & 90° 500 nm, 700 nm 12 OV 0°, 135° 550 nm, 650nm

TABLE 3 Hue ratios measured by the computerised sensor for differentinks. Paper Type h₄₅ h₉₀ h₁₁₀ White copy paper (matt) 1.0220 (±0.0106)1.0351 (±0.0127) 1.0331 (±0.0098) Sketch pad paper (matt) 1.0410(±0.0097) 1.0396 (±0.0081) 1.0644 (±0.0091) Pink calendar card (matt)1.2733 (±0.0048) 1.2401 (±0.0133) 1.1958 (±0.0178) Yellow calendar card(matt) 1.0636 (±0.0059) 1.0350 (±0.0071) 1.0261 (±0.0110) Orangecalendar card (matt) 1.7290 (±0.0139) 1.5196 (±0.0142) 1.5186 (±0.0246)Beige calendar card (matt) 1.0095 (±0.0037) 1.0121 (±0.0077) 1.0164(±0.0112) Grey calendar card (matt) 1.0058 (±0.0044) 0.9811 (±0.0079)0.9617 (±0.0166) Light blue card (matt) 0.9348 (±0.0013) 0.9519(±0.0036) 0.9565 (±0.0052) Light green card (matt) 0.9464 (±0.0043)0.9738 (±0.0046) 0.9843 (±0.0049) Dark blue folder card (matt) 0.9563(±0.0133) 0.9281 (±0.0109) 1.0577 (±0.0142) Green folder card (matt)0.9510 (±0.0081) 0.9492 (±0.0087) 0.9660 (±0.0095) Brown folder card(matt) 1.1789 (±0.0116) 1.1125 (±0.0035) 1.1142 (±0.0124) Yellow pagespaper (matt) 1.0841 (±0.0248) 1.0494 (±0.0306) 1.0566 (±0.0291) Newspaper white (matt) 1.0179 (±0.0282) 0.9972 (±0.0218) 1.0204 (±0.0280)Envelope brown (matt) 1.1232 (±0.0067) 1.0581 (±0.0190) 1.1086 (±0.0065)Maroon thesis cover (matt) 3.6571 (±0.0226) 2.6571 (±0.0485) 2.7122(±0.0214) Light red book cover (glossy) 3.5824 (±0.0341) 1.7056(±0.1111) 3.3564 (±0.1539) Dark green book cover (glossy) 0.5479(±0.0131) 0.7087 (±0.2240) 0.7391 (±0.0559) Syquest blue cover (glossy)1.0000 (±0.0367) 1.0418 (±0.0132) 1.1304 (±0.0814) Light green RS cover(glossy) 0.5000 (±0.0058) 0.8775 (±0.1852) 0.7179 (±0.0491) Yellow pagescover (glossy) 1.0128 (±0.0128) 0.9191 (±0.0353) 1.0391 (±0.0162)Aluminium casing (brushed) 0.9662 (±0.0124) 0.9149 (±0.0164) 1.0333(±0.0134) Beige computer casing (glossy) 0.9752 (±0.0100) 0.9416(±0.0256) 0.9905 (±0.0127) Dark blue book (glossy) 1.0909 (±0.0719)0.8426 (±0.0457) 1.0513 (±0.0443) Red RS cover (glossy) 6.8876 (±0.1043)1.7767 (±0.0990) 6.6304 (±0.2472) Blue RS cover (glossy) 0.5966(±0.0109) 0.9888 (±0.0583) 0.6552 (±0.0287) White book (glossy) 0.9925(±0.0049) 1.0205 (±0.0160) 1.0000 (±0.0034) Bright red glossy leaflet(glossy) 4.4338 (±0.1149) 1.7085 (±0.0627) 4.1333 (±0.1317) Sample #1(OVM) 1.7432 (±0.0354) 0.8049 (±0.0215) 0.6916 (±0.0205) Sample #2 (OVM)0.6244 (±0.0108) 0.4919 (±0.0110) 0.6801 (±0.0154) Sample #3 (OVM)1.8609 (±0.0425) 0.7646 (±0.0228) 0.6472 (±0.0167) Sample #4 (Metallicpaint) 1.2685 (±0.0238) 1.0945 (±0.0309) 1.3196 (±0.0320) Sample #5(Pearlescent ink) 1.0458 (±0.0104) 0.8622 (±0.0127) 0.9492 (±0.0120)Sample #6 (Pearlescent ink) 1.0690 (±0.0136) 1.0349 (±0.0130) 1.0562(±0.0142) Sample #7 (Pearlescent ink) 1.0465 (±0.0114) 0.8832 (±0.8832)0.9994 (±0.0170) Sample #8 (Pearlescent ink) 1.0260 (±0.0077) 1.0450(±0.0127) 1.0897 (±0.0111) Sample #9 (Pearlescent ink) 1.0181 (±0.0106)1.0325 (±0.0259) 1.0864 (±0.0154) Sample #10 (Pearlescent ink) 1.0024(±0.0090) 0.9907 (±0.0266) 1.0543 (±0.0171) Sample #11 (Pearlescent ink)0.9759 (±0.0095) 0.7457 (±0.0152) 0.9062 (±0.0121) Sample #12 (BanknoteOVM) 1.2573 (±0.0428) 0.8908 (±0.0168) 0.8872 (±0.0259) (Standarddeviations in brackets)

1. A method of determining if a surface contains a specific OVMcomprising the steps of: (1) illuminating the surface using lightcontaining two substantially monochromatic components of wavelength λ₁and λ₂, (2) detecting the intensity of scattered light from the surfaceat two scattering angles Ø₁ and Ø₂ selected according to the OVM ofinterest, (3) computing the magnitude of the difference between theintensity values for λ₁ light at Ø₁ and Ø₂ less the difference betweenthe intensity values for λ₂ light at Ø₁ and Ø₂, and (4) generating anoutput signal indicating the presence or absence of the specific OVMdepending on the computed difference magnitude relative to apredetermined value, wherein the surface is illuminated separately firstwith monochromatic light of one wavelength and then with monochromaticlight of the other wavelength, and wherein detection is performed by asingle detector which is moved between two positions so as to receivelight reflected/scattered from the surface first at one and then theother of the two angles Ø₁ and Ø₂.
 2. A method as claimed in claim 1,wherein the computed difference magnitude is compared with apredetermined value to generate a first output signal value indicatingthe presence of the specific OVM, if the computed magnitude is at leastas great as the predetermined value, or a second output signal valueindicating that the specific OVM has not been detected, if the computedmagnitude is less than the predetermined value.
 3. A method as claimedin claim 1, wherein the surface is illuminated by the two monochromaticcomponents simultaneously.
 4. A method as claimed in claim 1, whereindetection is performed by means of two detectors which are positioned soas to receive light from the surface along the directions dictated by Ø₁and Ø₂.
 5. A method as claimed in claim 1, which includes the step ofmodifying the values of signals from the one detector by signalamplification of the detector output signals at the two differentpositions to take account of any inherent differences in intensity ofthe originating illuminations incident on the surface due for example todifferent intensity levels and/or any misalignment of the sources of theλ₁ and λ₂ light.
 6. A method as claimed in claim 4, wherein the valuesof the signals from one or both detectors is modified by signalamplification to take account of any inherent differences in theresponses of the two detectors to light of given intensity incidentthereon, and any misalignment of the detectors.
 7. A method ofdetermining if a surface contains a specific OVM comprising the stepsof: (1) illuminating the surface using light containing twosubstantially monochromatic components of wavelength λ₁ and λ₂, (2)detecting the intensity of scattered light from the surface at twoscattering angles Ø₁ Ø₂ selected according to the OVM of interest, (3)computing the magnitude of the difference between the intensity valuesfor λ₁ light at Ø₁ and Ø₂ less the difference between the intensityvalues for λ₂ light at Ø₁ and Ø₂, and (4) generating an output signalindicating the presence or absence of the specific OVM depending on thecomputed difference magnitude relative to a predetermined value, andincluding a calibration procedure in which the light is projectedtowards and the reflected/scattered light is received from, a non OVMcontaining matt white surface.
 8. A method of determining if a surfacecontains a specific OVM comprising the steps of: (1) illuminating thesurface using light containing two substantially monochromaticcomponents of wavelength λ₁ and λ₂, (2) detecting the intensity ofscattered light from the surface at two scattering angles Ø₁ and Ø₂selected according to the OVM of interest, (3) computing the magnitudeof the difference between the intensity values for λ₁ light at Ø₁ and Ø₂less the difference between the intensity values for λ₂ light at Ø₁ andØ₂, and (4) generating an output signal indicating the presence orabsence of the specific OVM depending on the computed differencemagnitude relative to a predetermined value, and in which the absolutevalue of the computed difference is compared with a range of possiblevalues, the different values in the range corresponding to differingconcentrations of the specific OVM in or on the surface under test.
 9. Amethod as claimed in claim 1, wherein the light projected onto thesurface is collimated.
 10. A method as claimed in claim 4, wherein thesignals from each detector are gated or addressed in synchronism withthe changing wavelength of the illuminating light, so that during eachgating or addressing period the wavelength of the incident light isknown and there is no light of the other wavelength present to confusematters.
 11. A method of determining if a surface contains a specificOVM comprising the steps of: (1) illuminating the surface using lightcontaining two substantially monochromatic components of wavelength λ₁and λ₂, (2) detecting the intensity of scattered light from the surfaceat two scattering angles Ø₁ Ø₂ selected according to the OVM ofinterest, (3) computing the magnitude of the difference between theintensity values for λ₁ light at Ø₁ and Ø₂ less the difference betweenthe intensity values for λ₂ light at Ø₁ and Ø₂, and (4) generating anoutput signal indicating the presence or absence of the specific OVMdepending on the computed difference magnitude relative to apredetermined value, and wherein the difference magnitude M is computedin a way which compensates for variations in intensity between onewavelength component and the other, variations due to misalignment, andvariations between detector responses, using the following equation:M=|(K ₁ R ₁ −K ₂ R ₂)−A*(K ₁ G ₁ −K ₂ G ₂)| where R and G represent thereflectance signal intensity values outputted by the photo-detectors atthe illuminations of λ₁ and λ₂ respectively, the subscripts of R and Gdenoting measurements made by the two photo-detectors at the scatteringangles of Ø₁ and Ø₂, and K₁ and K₂ are calibration constants to allowfor misalignment and differences in the detector responses at Ø₁ and Ø₂scattering angles, and A is a scalar constant which normalises thedetector responses and alignment relative to the λ₁ and λ₂illuminations.
 12. A method as claimed in claim 11, wherein thecalibration constants K₁ and K₂ are set by adjusting the gain of thedetector output signal amplification.
 13. A method as claimed in claim11, wherein generation of a YES/NO output signal is achieved bycomparing the computed value of M with a predetermined value T, itselfderived by computing M from a surface containing a known minimumconcentration of the OVM of interest.
 14. A method as claimed in claim11 further comprising the step of determining the value of M for each ofa plurality of samples each containing the same OVM but at differentconcentrations per unit area, and storing same as a range of values ofT, for comparison with computed values of M from surfaces having anunknown concentration of the OVM thereon.
 15. A method as claimed inclaim 11 in which calibrating the detector involves the steps ofinserting a plain matt white surface in place of the surface to betested, and adjusting the values of K₁ and K₂ so that K₁R₁=K₂R₂=1, andadjusting scalar A to give M=0.
 16. A method as claimed in claim 15comprising the steps of substituting the white surface with a samplehaving a printed or coated surface, and checking for the presence of thespecific OVM, by comparing the computed magnitude of M with thepredetermined threshold value T, and generating a YES output signal ifthe magnitude of M is equal to or greater than T and a NO output signalif the magnitude of M is less than the threshold T.
 17. A method asclaimed in claim 11 further comprising the steps of storing differentvalues of T corresponding to different known OVM, for use with differentcombinations of λ₁, λ₂, Ø₁, and Ø₂, each unique to a particular OVM. 18.Apparatus by which an output signal is generated indicative of thepresence of a specific OVM in or on a surface under test comprising: 1.a light source which produces and projects along a projection axismonochromatic light at each of two wavelengths λ₁ and λ₂ selectedaccording to the specific OVM of interest;
 2. means for locating thesurface under test to receive the light with the projection axis at aspecific angle to the surface;
 3. two photodetectors, the first of whichis located to receive light reflected at a first scattering angle Ø₁,and the second of which is located so as to receive light reflected at asecond scattering angle Ø₂, from the surface, each photodetectorproducing an analogue signal indicative of the intensity of lightincident thereon;
 4. means for amplifying the signals from thephotodetectors;
 5. means for computing the value of the magnitude of thedifference between the amplified intensity values for λ₁ lightreflected/scattered from the surface at Ø₁ and Ø₂ less the differencebetween the amplified intensity values for λ₂ light at Ø₁ and Ø₂; 6.means for generating an output signal dependent on the magnitude of thecomputed difference value to indicate the presence of the material onthe surface, and wherein the illumination intensities and/or the gainsof the amplifying means are adjusted to calibrate the apparatus, duringa calibration step.
 19. Apparatus by which an output signal is generatedindicative of the presence of a specific OVM in or on a surface undertest comprising:
 1. a light source which produces and projects along aprojection axis monochromatic light at each of two wavelengths λ₁ and λ₂selected according to the specific OVM of interest;
 2. means forlocating the surface under test to receive the light with the projectionaxis at a specific angle to the surface;
 3. two photodetectors, thefirst of which is located to receive light reflected at a firstscattering angle Ø₁, and the second of which is located so as to receivelight reflected at a second scattering angle Ø₂, from the surface, eachphotodetector producing an analogue signal indicative of the intensityof light incident thereon;
 4. means for amplifying the signals from thephotodetectors;
 5. means for computing the value of the magnitude of thedifference between the amplified intensity values for λ1 lightreflected/scattered from the surface at Ø₁ and Ø₂ less the differencebetween the amplified intensity values for λ₂ light at Ø₁ and Ø₂; 6.means for generating an output signal dependent on the magnitude of thecomputed difference value to indicate the presence of the material onthe surface. and, wherein the light source comprises a pair of LED's,one of which emits near monochromatic light at or near λ₁ and the otherof which emits near monochromatic light at or near λ₂, and the lightfrom the two LED's is projected along a common axis.
 20. Apparatus asclaimed in claim 18 wherein the angle of incidence of the light upon thesurface is at or close to 45 degrees.
 21. Apparatus as claimed in claim18 further comprising collimating means by which the projected light iscollimated.
 22. Apparatus as claimed in claim 18 wherein thephoto-detector means comprises a pair of photo-diodes.
 23. Apparatus asclaimed in claim 22 wherein each photo-detector is associated with alens for focusing light onto the diode.
 24. Apparatus as claimed inclaim 23 wherein the lens has an aperture which is optimised to limitthe angular range of scattered light incident on the detector, butallows appropriate and practicable levels of light through to thephoto-detector.
 25. Apparatus as claimed in claims 19 further comprisinga pulsed power supply for powering the two LED's such that the two LED'sare operated alternately.
 26. Apparatus as claimed in claim 25 whereinthe repetition rate of the two LED's is greater than 1 KHz. 27.Apparatus as claimed in claim 26 wherein the repetition rate is lessthan 1 MHz.
 28. Apparatus as claimed in claim 18 wherein the apparatusand computing means are electronically hard wired, or the detector meanssupplies signals to, and is controlled by, a computer with a suitableinterface and data acquisition card.
 29. Apparatus as claimed in claim18 comprising analogue signal amplifying means and an analogue todigital converter (ADC) for supplying digital signals to the computingmeans.
 30. Apparatus as claimed in claim 29 wherein the analogue signalamplifying means and the ADC are provided on the interface or on thedata acquisition card or in the apparatus.
 31. Apparatus by which anoutput signal is generated indicative of the presence of a specific OVMin or on a surface under test comprising:
 1. a light source whichproduces and projects alone a projection axis monochromatic light ateach of two wavelengths λ₁ and λ₂ selected according to the specific OVMof interest;
 2. means for locating the surface under test to receive thelight with the projection axis at a specific angle to the surface; 3.two photodetectors, the first of which is located to receive lightreflected at a first scattering angle Ø₁, and the second of which islocated so as to receive light reflected at a second scattering angleØ₂, from the surface, each photodetector producing an analogue signalindicative of the intensity of light incident thereon;
 4. means foramplifying the signals from the photodetectors;
 5. means for computingthe value of the magnitude of the difference between the amplifiedintensity values for λ₁ light reflected/scattered from the surface at Ø₁and Ø₂ less the difference between the amplified intensity values for λ₂light at Ø₁ and Ø₂;
 6. means for generating an output signal dependenton the magnitude of the computed difference value to indicate thepresence of the material on the surface, and further comprisingcomparison means for generating a YES/NO signal indicating if theparticular OVM is present if the computed difference value is greaterthan a predetermined value obtained using a test sample surface having aknown concentration of the OVM therein or thereon.
 32. Apparatus bywhich an output signal is generated indicative of the presence of aspecific OVM in or on a surface under test comprising:
 1. a light sourcewhich produces and projects along a projection axis monochromatic lightat each of two wavelengths λ₁ and λ₂ selected according to the specificOVM of interest;
 2. means for locating the surface under test to receivethe light with the projection axis at a specific angle to the surface;3. two photodetectors, the first of which is located to receive lightreflected at a first scattering angle Ø₁, and the second of which islocated so as to receive light reflected at a second scattering angleØ₂, from the surface, each photodetector producing an analogue signalindicative of the intensity of light incident thereon;
 4. means foramplifying the signals from the photodetectors;
 5. means for computingthe value of the magnitude of the difference between the amplifiedintensity values for λ1 light reflected/scattered from the surface at Ø₁and Ø₂ less the difference between the amplified intensity values for λ₂light at Ø₁ and Ø₂;
 6. means for generating an output signal dependenton the magnitude of the computed difference value to indicate thepresence of the material on the surface, and further comprising memorymeans having stored therein a range of possible values for the computeddifference magnitude corresponding to different concentrations per unitarea of the specific OVM of interest in or on the surface and comparatormeans whereby the computed difference magnitude for an unknown surfaceis compared with the range of values in the memory means to determinethe best match and from a look-up table the concentration of the OVM onthe surface.
 33. A method of determining if a surface contains aspecific material comprising the steps of: (1) illuminating the surfaceusing light containing two substantially monochromatic components ofwavelength λ₁ and λ₂ (2) detecting the intensity of the light reflectedor scattered by the surface at first, second and third angles Ø₁, Ø₂ andØ₃, such that Ø₁ corresponds to back scattered light, Ø₂ corresponds toa near specular reflection, and Ø₃ corresponds to light leaving thesurface at a glancing scattering angle, (3) generating three outputsignals by computing hue ratios h_(Ø1), h_(Ø2) and h_(Å3), being theratio of the pairs of intensity values from each of the three detectorsfor the two monochromatic λ₁ and λ₂ components of illumination, and (4)comparing the computed hue ratios with a predetermined group of threestored values, obtained by experiment, to generate a final output signalwhose value depends on the comparison.
 34. A method as claimed in claim33 wherein the intensity values from the photo-detectors are adjusted tocompensate for background light by measuring and storing thephoto-detector output signal value when a surface is present but no λ₁or λ₂ illumination is incident thereon, and deducting the stored valuefrom intensity values produced by the photo-detectors when subjected toλ₁ or λ₂ reflected/scattered light.
 35. A method as claimed in claim 33wherein the angles are selected as being Ø₁=45°, Ø₂=90° and Ø₃=110°. 36.A method as claimed in claim 35 wherein values of λ₁ and λ₂ are selectedso that different groups of three hue values will arise depending onwhether the coating or ink in or on a surface under test is apearlescent OVM, a non-pearlescent OVM, or a non-OVM substance, and inthe case of the latter to distinguish between matt and glossy coatings.37. A method as claimed in claim 36 wherein two values for λ₁ and λ₂which enable such identification to occur are: λ₁=654 nm and λ₂=574 nm.38. A method of determining the material present in or on a surface forwhich three hue ratios h_(Ø1), h_(Ø2) and h_(Ø3) are determined usingthe method of claim 33 as between a matt ink, a glossy ink, an OVM ink,and a pearlescent OVM ink by checking the hue ratios against thefollowing criteria: (i) h_(Ø1), h_(Ø2) and h_(Ø3) are substantiallyconstant with scattering angle (indicating a matt ink), (ii) h_(Ø1),h_(Ø2) and h_(Ø3) tend to unity (indicating a glossy ink), (iii) h_(Ø1),h_(Ø2) and h_(Ø3) decrease with increasing scattering angle, and thedecreases tend to be substantial (indicating an OVM ink), (iv) specularreflection comprises a saturation colour and the h_(Ø2) ratio divergesfrom unity (indicating a pearlescent OVM ink).
 39. A method as claimedin claim 33 in which the angle of incidence of the illuminating light onthe surface is at or near 45°.
 40. A method as claimed in claim 39wherein the illuminating light is collimated.
 41. Apparatus adapted toperform the method of claim 33, comprising: (1) a light source whichproduces and projects along a projection axis monochromatic light ateach of two wavelengths λ₁ and λ₂, (2) platform means for locating asurface under test thereon to receive the light, with the projectionaxis at a specific angle to the surface, (3) three photo-detectorslocated relative to the platform so as to separately receivereflected/scattered light from a surface thereon at three differentangles Ø₁, Ø₂, and Ø₃, where Ø₁ corresponds to back scattered light, Ø₂to near specular reflection and Ø₃ to light leaving the surface at ashallow angle (a glancing scattering angle), (4) means for amplifyingthe signals from the photo-detectors, (5) means for computing the ratioof the response of each photo-detector to the two different wavelengthsin the reflected/scattered light incident thereon after takingbackground light into account, (6) comparator means for comparing thethree ratio values so obtained with at least one set of three storedvalues, and generating an output signal dependent on the comparison. 42.Apparatus as claimed in claim 41 wherein the photo-detector outputsignals are adjusted for background illumination before the hue ratiosare computed.
 43. Apparatus as claimed in claim 42 wherein backgroundintensity level signal values are obtained by noting each photo-detectoroutput signal value with the surface in place but when no λ₁ or λ₂illumination is present, and means is provided for storing thebackground intensity level output signals.
 44. Apparatus as claimed inclaim 42 further comprising computing means whereby the stored value isdeducted from subsequent output signals from each photo-detectorobtained signal value when the surface is illuminated by λ₁ and λ₂illumination.
 45. Apparatus as claimed in claim 41 in which thedetectors, light source and computing means are electronically hardwired.
 46. Apparatus as claimed in claim 41 wherein the illuminating anddetecting means are hard wired and supply signals to, and are controlledby, a computer with a suitable interface and data acquisition card. 47.Apparatus as claimed in claim 41 further comprising analogue signalamplifying means for amplifying intensity signals from thephoto-detectors.
 48. Apparatus as claimed in claim 47 further comprisinganalogue to digital converter (ADC) means adapted to convert theamplified signals to digital signals for computation by the computingmeans and/or storage.
 49. Apparatus as claimed in claim 41 wherein thecomparator means generates either a YES/NO signal in response to thecomparison depending on whether or not the three computed values aresimilar to three stored values, or an identification signal indicatingwhich of a plurality of groups of stored values (each comprising a groupof three such values) the three computed values most closely correspond.50. A method of identifying the presence of a particular type ofmaterial on a printed or coated surface, comprising the steps of: 1.illuminating the surface at a pre-set angle to the surface withsubstantially monochromatic light at three wavelengths λ₁, λ₂ and λ₃selected in accordance with the particular type of material, 2.detecting light reflected from the surface at three different angles Ø₁,Ø₂ and Ø₃, one of which Ø₂ corresponds to near specular reflection andthe other two of which are selected in accordance with the particulartype of material and are at or near to those at which the illuminationwavelengths give good reflectance changes for the particular type ofmaterial,
 3. computing three hue values from the intensity valuesdetermined by each detector for each of the three monochromaticillumination components λ₁, λ₂ and λ₃ respectively, thereby to producenine hue values relating to the surface,
 4. comparing the nine huevalues so obtained with a stored group of nine hue values, obtained byperforming the method on a surface containing a particular concentrationof the particular type of material, and
 5. generating a final outputsignal whose value depends on the comparison.
 51. A method as claimed inclaim 50 wherein the hue values are computed after taking into accountand adjusting the photo-detector output signals for any backgroundillumination.
 52. A method as claimed in claim 51 wherein the outputsare corrected for background illumination by noting the photo-detectoroutput signals with the surface present but in the absence of any λ₁, λ₂or λ₃ illumination, and storing the output signals for each of the threedetectors for deduction from the output signal values for the respectivedetectors when the surface is illuminated by the λ₁, λ₂ and λ₃illuminations.
 53. A method as claimed in claim 50 wherein the finaloutput signal is a binary signal and has one value only if identity ornear identity is obtained by the comparison, thereby indicating that theparticular type of material is present.
 54. A method as claimed in claim50 wherein the comparison is performed by calculating the nearestneighbour classifier using the nine stored hue values for the particularmaterial.
 55. A method as claimed in claim 54 wherein the nearestneighbour classifier is computed by summing the squares of thedifferences between the computed hue values and stored hue values andcomparing the sum with a threshold, wherein in the case of identity thevalue of the sum is zero and near identity situations are identified ifthe magnitude of the sum is less than a small numerical value selectedfor the threshold.
 56. A method as claimed in claim 50 whereincomputation of the nine hue values r_(φ1) g_(φ1) etc., for the threedetectors receiving reflected/scattered light at the angles Ø₁ and Ø₂and Ø₃ at the wavelengths λ₁, λ₂ and λ₃, each of which produces anintensity value in the detector output of R_(φ1), G_(φ1), B_(φ1),R_(φ2), G_(φ2), B_(φ2), R_(φ3), G_(φ3), and B_(φ3) respectively, andwhere the background illumination produces an intensity value D_(φ1),D_(φ2) etc., in the detector output, is achieved by using the equations.r _(φ1)=(R _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))g _(φ1)=(G _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))b _(φ1)=(B _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))r _(φ2)=(R _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))g _(φ2)=(G _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))b _(φ2)=(B _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))r _(φ3)=(R _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3))g _(φ3)=(G _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3))b _(φ3)=(B _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3)) whereR_(φ1,2,3) are the λ₁ illuminated intensity signals from detectors atscattering angles of Ø₁, Ø₂, and Ø₃ respectively; G_(φ1,2,3) andB_(φ1,2,3) are the same but for λ₂ and λ₃ illumination respectively. 57.A method of calibrating apparatus comprising the steps of performing themethod of claim 56 using a surface containing a given concentration ofthe particular material of interest and computing the 9 values r_(φ1),g_(φ1) etc., and storing the computed values as α₁, α₂, α₃, β₁, β₂, β₃,γ₁, γ₂, γ₃.
 58. A method as claimed in claim 57 wherein the method isrepeated and the results stored for each of a plurality of surfacescontaining different known inks/coatings.
 59. A method of computing anearest neighbour value E using values obtained from the equationsr _(φ1)=(R _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))g _(φ1)=(G _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))b _(φ1)=(B _(φ1) −D _(φ1))/(R _(φ1) +G _(φ1) +B _(φ1)−3D _(φ1))r _(φ2)=(R _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))g _(φ2)=(G _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))b _(φ2)=(B _(φ2) −D _(φ2))/(R _(φ2) +G _(φ2) +B _(φ2)−3D _(φ2))r _(φ3)=(R _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3))g _(φ3)=(G _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3))b _(φ3)=(B _(φ3) −D _(φ3))/(R _(φ3) +G _(φ3) +B _(φ3)−3D _(φ3)) whereR_(φ1,2,3) are the λ₁ illuminated intensity signals from detectors atscattering angles of Ø₁, Ø₂, and Ø₃ respectively; G_(φ1,2,3) andB_(φ1,2,3) are the same but for λ₂ and λ₃ illumination respectively, andvalues stored using the results of the method of claim 58 by using theequation:E = (r_(ϕ 1) − α₁)² + (g_(ϕ 1) − β₁)² + (b_(ϕ 1) − γ₁)²+    (r_(ϕ 2) − α₂)² + (g_(ϕ 2) − β₂)² + (b_(ϕ 2) − γ₂)²+    (r_(ϕ 3) − α₃)² + (g_(ϕ 3) − β₃)² + (b_(ϕ 3) − γ₃)².60. A method of determining if a particular material is present in or ona surface under test comprising the step of computing E by the method ofclaim 59 and noting the value of E.
 61. A method of determining which ofa plurality of possible materials exist in or on a surface comprisingpre-storing different groups of α, β, γ values for differentpre-measured samples and performing nearest neighbour thresholdclassification using each of the stored α, β, γ value sets, in theequation of claim 59, until the lowest value of the sum is obtained,indicating the best match, and generating an output signal indicative ofthe material characteristic of the group of α, β, γ values soidentified.
 62. Apparatus adapted to perform the method of claim 60comprising: (1) three monochromatic light sources producing light of λ₁λ₂ and λ₃ wavelengths, the particular wavelengths being selected inrelation to the material to be identified, (2) means for projecting thelight at a particular angle towards a support means on which a sheet ofmaterial the surface of which is to be investigated can be laid, theangle being selected in relation to the material to be identified, (3)three photo-detectors arranged relative to the support means to receivereflected/scattered light along three different directions therefrom,the directions being selected in relation to the material to beidentified; (4) computing means adapted to receive intensity signalsfrom the three detectors and compute therefrom nine hue valuescorresponding to ratios of intensity signal values and combinations ofsuch signal values, from each detector; (5) memory means adapted tostore at least one set of nine hue values obtained by using a sheet ofmaterial containing at least in or on the surface thereof the materialwhich is to be looked for in other surfaces, (6) comparison means forcomparing computed and stored hue values to generate an output signaldepending on the comparison.
 63. Apparatus as claimed in claim 62wherein the hue values are computed after taking into account andadjusting the photo-detector output signals for, any backgroundillumination.
 64. Apparatus as claimed in claim 62 wherein thecomparison means comprises a computing means adapted to compute the sumof the squares of the differences between the computed and stored huevalues.
 65. Apparatus as claimed in claim 62 wherein the projectionangle is 45°.
 66. Apparatus as claimed in claim 62 further comprisingcollimating means to collimate the projected light.
 67. Apparatus asclaimed in claim 62 wherein each photo-detector comprises a photo-diode.68. Apparatus as claimed in claim 67 wherein lens means is provided forfocusing reflected/scattered light from the surface onto eachphoto-diode.
 69. Apparatus as claimed in claim 68 wherein the lens meanshas an aperture which is selected so as to limit the angular range ofscattered light which will reach its associated photo-detector. 70.Apparatus as claimed in claim 62 wherein each of the monochromatic lightsources is an LED.
 71. Apparatus as claimed in claim 62 which includes apulsed power supply for the LED's, such that the three LED's areoperated alternately in series, with an off period to allow backgroundlight to be measured.
 72. Apparatus as claimed in claim 71 wherein therepetition rate of the LED's 1 KHz.
 73. Apparatus as claimed in claim 71wherein the repetition rate is less than 1 MHz.
 74. A method ofdetermining the presence of a particular material especially an OVM onor in a surface comprising performing any two of the methods ofdetermining if a surface contains a specific OVM comprising the stepsof: (1) illuminating the surface using light containing twosubstantially monochromatic components of wavelength λ₁ and λ₂, (2)detecting the intensity of scattered light from the surface at twoscattering angles Ø₁ and Ø₂ selected according to the OVM of interest,(3) computing the magnitude of the difference between the intensityvalues for λ₁ light at Ø₁ and Ø₂ less the difference between theintensity values for λ₂ light at Ø₁ and Ø₂, and (4) generating an outputsignal indicating the presence or absence of the specific OVM dependingon the computed difference magnitude relative to a predetermined value,on the surface and noting the results obtained from the two methods, andgenerating a final output signal indicating the material is in or on thesurface depending on the results obtained from the two methods, whereinthe two methods are performed in parallel so that the results obtainedby performing the two methods are available at substantially the samepoint in time to enable a final classification signal to be produced.75. A method of determining the presence of a particular materialespecially an OVM on or in a surface comprising performing any two ofthe methods of determining if a surface contains a specific OVMcomprising the steps of: (1) illuminating the surface using lightcontaining two substantially monochromatic components of wavelength λ₁and λ₂, (2) detecting the intensity of scattered light from the surfaceat two scattering angles Ø₁ and Ø₂ selected according to the OVM ofinterest, (3) computing the magnitude of the difference between theintensity values for λ₁ light at Ø₁ and Ø₂ less the difference betweenthe intensity values for λ₂ light at Ø₁ and Ø₂, and (4) generating anoutput signal indicating the presence or absence of the specific OVMdepending on the computed difference magnitude relative to apredetermined value, on the surface and noting the results obtained fromthe two methods, and generating a final output signal indicating thematerial is in or on the surface depending on the results obtained fromthe two methods, wherein the two methods are performed one after theother in quick succession, and the result(s) of the first method to beperformed is/are stored for combining with the result(s) from the secondmethod to be performed, to produce the final classification signal. 76.A method of article authentication wherein genuine articles are coatedover at least part of their surface with a known OVM comprising thesteps of: (1) illuminating the surface using light containing twosubstantially monochromatic components of wavelength λ₁ and λ₂, (2)detecting the intensity of scattered light from the surface at twoscattering angles Ø₁ and Ø₂ selected according to the OVM of interest,(3) computing the magnitude of the difference between the intensityvalues for λ₁ light at Ø₁ and Ø₂ less the difference between theintensity values for λ₂ light at Ø₁ and Ø₂, and (4) generating an outputsignal indicating the presence or absence of the specific OVM dependingon the computed difference magnitude relative to a predetermined value,and selecting the scattering and photo-detector angles and thewavelengths of the illuminating light in accordance with the OVM whichshould be on the article, and comparing the output signal value producedby the method against a look-up table (having at least one valuetherein) to generate an authentication or rejection signal depending onthe value of the output signal.
 77. A method as claimed in claim 76wherein the article comprises a passport, an ID card, a driving licence,a banknote, a bond, a share certificate, a postage stamp or any othersecurity document.
 78. A method as claimed in claim 76 wherein only partof the surface carries the OVM and the area of the spot of light formedthereon by the illuminating light is no larger than the said area.
 79. Amethod as claimed in claim 78 wherein at least the said area of thesurface is maintained flat and at an appropriate angle to theilluminating light beam.
 80. A method of checking the quality of theprinting or coating of at least part of a substrate surface so as todeposit a particular OVM thereon, comprising the step of illuminatingthe part of the surface which has been printed or coated with the OVMand investigating the reflected light in accordance with a method asclaimed in claim 1 and generating a pass or fail signal depending on thevalue of the output signal generated by the method.
 81. A method asclaimed in claim 80 wherein the printed substrate is sheet materialwhich has been or is being, or will be printed, to form one or moresecurity documents.
 82. A method as claimed in claim 81 wherein thesecurity documents are banknotes and the OVM is applied to some or allof the surface of the sheet material before, during or after it isprinted to create the banknotes.
 83. A method as claimed in either ofclaim 81 wherein the sheet material is moving relative to the LED andphotodiode detector assembly at a linear speed of the order of 20 metersper second.
 84. A printing or coating apparatus adapted to apply aspecific OVM to sheet material in combination with apparatus as claimedin claims 18 adapted to generate a pass or fail signal depending on thevalue of the output signal produced by the apparatus as printed orcoated sheet material moves relative thereto.
 85. A document sortingapparatus by which documents can be sorted according to whether or not aparticular OVM is present in the surface of each document in combinationwith apparatus as claimed in claim 18 adapted to generate a firstcontrol signal if the value of the output signal generated by theapparatus for a specific document indicates the particular OVM ispresent therein and a second control signal if the output signalindicates no OVM to be present, and supplying a route controlling signalto the routeing apparatus to route the specific document to one of twodestinations depending on whether a first or second control signal isgenerated by the apparatus.
 86. A method of detecting the presence of anon-pearlescent OVM in or on a surface, comprising the steps ofilluminating the surface at a first angle to the surface and detectingand determining the frequency spectrum of scattered light in twodifferent directions from the surface, one direction subtending an angleto the surface at a second angle which is substantially different fromthe said first angle and is substantially parallel to the plane of thesurface, and the other direction subtending an angle to the surface at athird angle which is substantially closer to the said first angle thanthe said one direction, and in which the angle of the one direction tothe surface at the second angle is in the range 1° to 15° and the anglemade by the other direction to the surface at the third angle is within10° of the said first angle.
 87. A method as claimed in claim 86,wherein the said second angle is 10° and the third angle equals the saidfirst angle.
 88. A method of determining the presence of a pearlescentOVM in or on a surface comprising the steps of illuminating the surfaceat a first angle and firstly detecting and determining the frequencyspectrum of substantially direct specular reflection from the surface,and secondly detecting and determining the frequency spectrum ofscattered light leaving the surface at an angle which is different fromthat at which direct specular reflection occurs, wherein the seconddetection is of forwardly scattered light, and wherein the forwardlyscattered light is detected at an angle to the said surface which is inthe range 1° to 15°.
 89. A method as claimed in claim 88, wherein theangle is 10°.
 90. A method as claimed in claim 88, wherein the seconddetection is of back scattered light.
 91. A method as claimed in claim90, wherein the back scattered light is detected at an angle within 10°of the direction in which illuminating light is projected towards thesurface.
 92. A method as claimed in claim 91 wherein the back scatteredlight is detected at substantially the same angle as that which theilluminating light makes to the said surface.
 93. A method as claimed inclaim 86, wherein the two detections are performed simultaneously.
 94. Amethod as claimed in claim 86, wherein the two detections are performedin succession one after the other.
 95. A method as claimed in claim 94,wherein a single detector is employed, which is moved between the twopositions to allow light which is being reflected from or scattered bythe surface in the different directions of interest to be intercepted.96. A method as claimed in claim 86, wherein a plurality of detectors isprovided each fixed in position to intercept reflected or scatteredlight as appropriate, and the detectors are separately interrogatedeither one after the other, to provide a succession of intensity values,or simultaneously to provide a corresponding plurality of intensityvalues.
 97. A method as claimed in claim 86, wherein the spectraldetermination of the light incident on the or each detector is performedby the detector by choice of a suitable photo-sensitive element or acombination of one or more filters and at least one photo-detector whichwill supply different signals or different values of a parameter of asignal, depending on the wavelength of light incident thereon.
 98. Amethod as claimed in claim 97, wherein the illuminating light is whitelight.
 99. A method as claimed in claim 86, wherein the illuminatinglight is made up of two or more distinct monochromatic components havingdifferent (known) wavelengths.
 100. A method of article authenticationwherein genuine articles are coated over at least part of their surfacewith a known OVM, comprising the steps of:
 1. illuminating the surfaceat a pre-set angle to the surface with substantially monochromatic lightat three wavelengths λ₁, λ₂ and λ₃ selected in accordance with theparticular type of material,
 2. detecting light reflected from thesurface at three different angles Ø₁, Ø₂ and Ø₃, one of which Ø₂corresponds to near specular reflection and the other two of which areselected in accordance with the particular type of material and are ator near to those at which the illumination wavelengths give goodreflectance changes for the particular type of material,
 3. computingthree hue values from the intensity values determined by each detectorfor each of the three monochromatic illumination components λ₁, λ₂ andλ₃ respectively, thereby to produce nine hue values relating to thesurface,
 4. comparing the nine hue values so obtained with a storedgroup of nine hue values, obtained by performing the method on a surfacecontaining a particular concentration of the particular type ofmaterial, and
 5. generating a final output signal whose value depends onthe comparison, and selecting the scattering and photo-detector anglesand the wavelengths of the illuminating light in accordance with the OVMwhich should be on the article, and comparing the output signal valueproduced by the method against a look-up table (having at least onevalue therein) to generate an authentication or rejection signaldepending on the value of the output signal.
 101. A method as claimed inclaim 100 wherein the article comprises a passport, an ID card, adriving licence, a banknote, a bond, a share certificate, a postagestamp or any other security document.
 102. A method as claimed in claim100 wherein only part of the surface carries the OVM and the area of thespot of light formed thereon by the illuminating light is no larger thanthe said area.
 103. A method as claimed in claim 102 wherein at leastthe said area of the surface is maintained flat and at an appropriateangle to the illuminating light beam.
 104. A method of checking thequality of the printing or coating of at least part of a substratesurface so as to deposit a particular OVM thereon, comprising the stepof illuminating part of the surface which has been printed or coatedwith the OVM and investigating reflected light in accordance with amethod comprising the steps of:
 1. illuminating the surface at a pre-setangle to the surface with substantially monochromatic light at threewavelengths λ₁, λ₂ and λ₃ selected in accordance with the particulartype of material,
 2. detecting light reflected from the surface at threedifferent angles Ø₁, Ø₂, and Ø₃, one of which Ø₂ corresponds to nearspecular reflection and the other two of which are selected inaccordance with the particular type of material and are at or near tothose at which the illumination wavelengths give good reflectancechanges for the particular type of material,
 3. computing three huevalues from the intensity values determined by each detector for each ofthe three monochromatic illumination components λ₁, λ₂ and λ₃respectively, thereby to produce nine hue values relating to thesurface,
 4. comparing the nine hue values so obtained with a storedgroup of nine hue values, obtained by performing the method on a surfacecontaining a particular concentration of the particular type ofmaterial, and
 5. generating a final output signal whose value depends onthe comparison, and generating a pass or fail signal depending on thevalue of the output signal generated by the method.
 105. A method asclaimed in claim 104 wherein the printed substrate is sheet materialwhich has been or is being, or will be printed, to form one or moresecurity documents.
 106. A method as claimed in claim 105 wherein thesecurity documents are banknotes and the OVM is applied to some or allof the surface of the sheet material before, during or after it isprinted to create the banknotes.
 107. A method as claimed in either ofclaim 105 wherein the sheet material is moving relative to the LED andphotodiode detector assembly at a linear speed of the order of 20 metersper second.